In which yours truly discovers a few monsters have been accidentally skipped over. Why, SRD? I trusted you!
I've been trying to make sense of the githyanki picture for several hours now. My best guess is that it's an intentional parody of all impractical armor ever.
That said, githyanki aren't that bad. Their ability adjustments are bland (if not that bad) and their SR low, but the scaling psionic abilities make up for a lot of that. It starts off with Daze (actually not all bad at low levels), moves on to Blur and Dimension Door, and ends with Telekinesis and Plane Shift. Note that all these abilities are gained at the levels a wizard or sorcerer would've gotten them.
Feat-wise, githyanki have a single notable perk. Githyanki Battlecaster is relatively easy to get and gives what's essentially Armored Mage. If desired, this can then be expanded upon with Battle Caster.
Githyanki need some kind of LA; that much is obvious. In the end, I'm going to go with +1, though I must say I've strongly considered +2.
Finally, please note that in campaigns going past level 16 being a githyanki gets far less desirable.
Githzerai have a better chassis than their astral brethren, with a huge dexterity boost, a smaller wisdom bonus, and only a minor intelligence penalty. Their SR is equal to the githyanki's, meaning it's not very good.
Psionics-wise, they're a bit behind. The low-level SLA's are better, but afterwards githzerai only get an upgrade once; at level 11 (two levels after githyanki) they access Plane Shift.
Githzerai also have a built-in armor bonus, which I suppose is nice for the monks and casters amongst them.
So what LA fits? I'm going to go with a strong +1 here, just like the githyanki's.
I had a lot of trouble coming up with appropriate LA's here, so I'd love for people to give their own opinions here.