Um... Do Demons Really Need More Power? Actually I'd say yes... the potential for such power, at least. Remember, though crippled from eons in the abyss and stuck in human bodies, the fallen are angels, Celestial beings of vast power. This isn't to say that player characters should necessarily wield such power; a Storyteller needs to have a very clear picture of what he's getting into before he allows the characters with Faith ratings of 10+ into his Demon chronicle, or handing out so much experience that such ratings become quickly attainable. Such big guns are probably best left in the hands of the Storyteller, to be perfectly honest, though it's his call if he wants a player character running around with seven dots of Lore of the Flame and the potential to incinerate a city block.

Demon doesn't have much information for adjudicating the capabilities of old, advanced characters. As it stands, the traits of Elohim cap at five dots, with characters having no ability to advance beyond this. While this shouldn't be a problem for most players, it makes sense that fallen strong in Faith can develop their mundane and supernatural skills to astonishing levels, as well as develop a higher tolerance for Torment. The fact that there are no systems to account for this (at least that I've seen) is more a credit to Demon: the Fallen than an actual oversight, as it reinforces the importance of the character over her stats.
But, I do feel that there should be some option to simulate characters with exceptional abilities. It only makes sense if a Devil warrior has been traveling the earth since the Crusades, and has developed her Faith to truly astounding degrees in the meantime, she'd have picked up her sixth or even seventh dot in Manipulation, Melee, Subterfuge or Influence long ago, and wield evocations the stuff of legend...
An older character has a better chance of having the requisite Faith to attain superhuman ratings in his traits and to unlock legendary Lore than some up-and-comer fresh from the Abyss. Storytellers might find the Age Background, detailed in Forged in Torment, useful.

How Can Such Powerful Demons Inhabit a Human Body?
Normally, very powerful demons spirits destroy any mortal body they attempt to inhabit in a matter of days, as evidenced by ancient Earthbound and greater demons from the Pit attempting to do so. How do the ancient fallen discussed here get around this? First, the host body is tempered by the massive amount of Faith channeled through it by the crippled angel within it, and gradually can more easily accommodate the massive Celestial energies forced upon it as the demon waxes in power. At the same time, the Sebettu becomes more accustomed to having a mortal body, and learns to expend its powers more safely. Very powerful demonic spirits forcing themselves into a host body don't have the time to adapt the body to them and vice-versa, and they quickly burn out their hosts. (This does means a powerful demon whose host is destroyed may well destroy the next human body he possesses, not having the years necessary to "break in" the shell. The unfortunate Sebettu would be pulled back to the Abyss, or perhaps become an Earthbound... though at the Storyteller's option, perhaps a drastic reduction in power might allow the fallen to again possess a human body.)
However, human flesh was never designed to house fallen angels, especially those that begin to approach their former levels of power. Something has to give. While the host body isn't destroyed, it undergoes drastic changes, and the fallen becomes unable to retain mortal form for very long, defaulting to the apocalyptic visage. Whether the Celestial embraces this transformation or reacts with horror, it presents some unique challenges in her life henceforth. If she doesn't wish to attract attention (and no demons of such power do), a monastic life is her only recourse. Assuming she dwelt among mortals before, it becomes almost impossible for her to do so now, making reaping Faith much more difficult. The fallen angel's inhuman visage also has a profound psychological impact on her. In addition to the ennui of ages and the resurfacing of memories, the demon is driven further from the world of mortals and events of the world, and often deeper into her Torment; see the Age Background for more on the unique challenges faced by elder Celestials.

Systems
Enough flavor text and Storyteller posturing? Fine. Here you go, the crunchy bits.
The capacity for exceptional ratings (6+) in Attributes, Abilities and Virtues is a function of the fallen's Faith, below. Lore may not be increased beyond 5; instead, demons with exceptional Faith may invest in the Mastery Background (see below). Note that that high Faith only provides the potential for supernaturally high traits, and isn't an entitlement to it; in other words, experience must be spent to increase traits, as normal. Note that experience point costs remain unchanged, making maxing out a character's Faith, much less her other traits, an expensive endeavor indeed.
One benefit of high Faith is an increased threshold for Torment; a demon with sufficient Faith can steel herself against her Torment, allowing greater excesses of pain and rage before losing herself to it completely. This is expressed through raising the cap for Torment; normally, a player loses control of his character after it attains a Torment of 10, but those with incredible stores of Faith have a little more leeway -- though not a lot -- and can tolerate higher Torment ratings. (However, fallen with Torment ratings of 10 and higher can't access non-Tormented evocations.)
However, there's a price for such awesome Faith. A demon with a Faith rating of 10 or higher is more Celestial than mortal, and therefore her "default" form is that of her revelatory visage, rather than her human form. To transform to human form, she must roll Willpower (difficulty equals her Faith, minus 8); each success allows her to retain her mortal form for one hour that day (only one roll may be made per 24-hour period). A single success on this roll is enough for a successful transformation, but with less than three successes some inhuman characteristics may bleed through; for example, an Ereshkigal Slayer might radiate the chill of a crypt and be swathed in light shadow, while a Rabisu with the Zaltu visage may possess yellow eyes and claw-like fingernails.
Finally, truly powerful demons discover it's difficult to go unnoticed. When a character with a Faith ratings of 10 or higher uses lore or any sort of Infernal power, other Sebettu can sense it at -1 difficulty (see Supernatural Awareness, pg. 172 of the core book). Fortunately for the fallen in question, her apocalyptic form isn't any easier than normal to detect than that of another demon... unfortunately, as remains in that form so much, she risks being detected through other demons' supernatural senses.

Faith Trait Cap Mastery Cap Torment Cap 10-11 6 1 11 12-14 7 2 12 15-18 8 3 13 19+ 9 4 14

Legendary Lore
Okay, the part you've all been waiting for, and what you probably skipped all the stuff above to read about. While a creative Storyteller or player can develop evocations from levels six to nine, that's bound to be a lot of work. A simpler way of handling advanced characters is to expand upon the lore they have.
If a demon's primary lore is 5 and he meets the Faith requirements (see table above), he has the option of purchasing one or more dots in the Background: Mastery, detailed in Earthbound. (See pgs. 76, 77 and 83 of that book for rules.) If this evolution happens in-game, the first Mastery dot cost 20 experience points and buying further levels costs current rating x 15. (Remember, Mastery is very powerful!) The benefits of Mastery can be applied to any lore in which the fallen has a rating of 5, though any evocation outside his primary lore are at +1 difficulty; rituals that employ the character's primary lore don't suffer this difficulty penalty. However, it's difficult for Sebettu within mortal frames to channel such massive Celestial energies.
All Faith costs are doubled, and the character will take a number of health levels of lethal damage equal to the the umber of Faith; fallen in apocalyptic form may soak this damage, however. Despite these severe penalties, the payoff is often worth it. (So a demon with Mastery 2 must spend 4 Faith if he wants to increase the range of an evocation a hundredfold, and he suffers four levels of lethal damage.)