Let's see... assuming a psion with no bonus PP's and no manifester-level increasing stuff, a Psion making maxed manifestations will make a limited number of them, dependant on level:
2 at level 1
3 at level 2
3 at level 3
4 at 4th
5 at 5th
5 at 6th
6 at 7th
7 at 8th
8 at 9th
8 at 10th
9 at 11th
10 at 12th
11 at 13th
12 at 14th
13 at 15th
13 at 16th
14 at 17th
15 at 18th
16 at 19th
and 17 at 20th.
Now, that looks like a lot (for comparison, a Sor 10 gets 3 level 5 spells by default), but a sorceror or wizard also gets lower-level spells included at relatively little expense - sure, that sorceror has 3 castings of his level 5 spell, while the psion can manifest 5th level powers 8 times - but the Sorceror can still cast his five level 4 spells, his 6 level 3 spells, and so on. Moreover, at 10th, that sorceror can cast six 10d6 fireballs and 3 10d6 cone of colds.... and STILL have room for his buffing spells (which are mostly 1st and second level). So the Sorceror gets 90d6 damage in 10d6 increments, while the Psion is only getting 80d6 damage in 10d6 increments, with virtually no PP's left over for buffs. Is a psion overpowered in an endurance campaign? No, not at all.
If you include metamagic and metapsionics, the psion with Psionic Meditation can burn and regain his focus (full-round equivalent, requires a roll vs. concentration) and spend an extra 2 pp's to make those 10d6's into 12d6's (8(for power directly) +2(for Empower) gives 8d6+4d6) eight times, for 104d6, all told. The Sorceror with Empower can turn those 3 Cone of Cold's into Empowered Fireballs for 15d6 each; 6 10d6 regular fireballs + 3 15d6 fireballs for 105d6 damage, all told. Meanwhile, the Psion has burned through all his power points while the Sorceror still has all his 4th level slots, 2nd level slots, 1st level slots, and 0th level slots open.... and he can spend his 4th level slots on more 10d6 fireballs, if he has a mind to do so, as well as spending those 2nd level slots on Scorching Ray (2 rays per casting, 4d6 each ray, 6 2nd level base slots) and those 1st level slots on Magic Missle (5 * 1d4+1 per casting, 6 1st level base slots). Sorceror beats Psion at endurance in raw damage dealing, at 10th, at least. Not sure about other levels, but I suspect it works out very similarly.
To be fair, a Psion might have Mind Thrust, and change all those d6's to d10's - in which case, he is potentially out-blasting the Sorceror in raw damage for just the sorceror's 5th and 3rd slots - of course, he adds that pesky mind-affecting tag, making many opponents immune.
Sure, the Psion has more flexibility than the Sorceror ... but less raw power.