Well, I did some Dark Sun and Planescape under 2nd Ed as a player. Here's my thoughts based on that:
I agree with Ciaran that the powers don't really scale well. It can fall one of two ways: either the peson doesn't really know what they're doing, and the character can never do anything, or they can twink the character out to the point where it can trounce the rest of the party.
The main problem is, at least for the Psionicist's Handbook, that the powers were pretty haphazard- they weren't really balanced against other classes, or each other. If a player (such as myself) were to happen upon certain deadly combinations, then that was it. For instance, the Double Pain power doubled the damage of a single attack to a person, although I believe it was technically illusory subdual damage. Several levels later, I took Amplify, to triple and eventually quadruple damage. Combined with x4 from my backstab ability (psionicist/assassin), things could get pretty ridiculous. Even if 80% of the damage were illusory, the enemy was still out cold and helpless.
Power point recovery was the other big issue I found- there were ways to recover power points very quickly, so that you could effectively use and recover your pool two or three times during the day. I can't give details, since I don't have the book any more.
One thing I will say- if you can get ahold of the second Dark Sun rules, take a look at those. They adapted the Psionicist's Handbook to a more usable format. MAC, for instance, never changed- but the Dark Sun psionicist was given a mental THAC0 that went up over time, so that eventually a higher-level psionicist actually could affect peasants more easily. There were probably other changes, too, but I don't have the rules.
That's my perspective- I don't really have any adaption ideas. At the risk of suggesting yet another third ed source (don't know if that's what your playing or not), you may want to check out Green Ronin's Psychic book:
http://www.greenronin.com/cgi-bin/product.cgi?prodid=1306
It's a skills and feats system, similar to force powers in Star Wars from what I've heard. As such it could be implemented outside of class systems, and might be what your looking for. You could make the entrance feat (I assume there's one) available only at first level to simulate the "must be born psychic" feel of your campaign.
Hope this helps!