I think the thing is when comparing wiz/sor to a psion you can't just look at the # of 9th level effects a psion will be throwing and compare it to the wiz/sor. The wiz/sor effects thanks to scaling of 7,8,9 will in effect be roughly equivelent to those psion 9th level effects. So the sorcerer will likely be throwing 20 of those level of effects and still have a wide assortment of effects from the lower levels many of which again thanks to scaling will be effective in an encounter.
Also at the lower levels it looks worse for the psion at level 10 for example IIRC a psion with a 18ish in a stat would throw about 10 effects costing 10PP. The wizard can be throwing 3,4,5 levels spells that would have an effect at a similar level and would be throwing 10 of them. The sor would be throwing even more and at these levels level 1 and 2 spells are still potent.
The one area I think they screwed up in balance was the bonus PP for an attribute. They should of just translated bonus spells over to bonus PP. They almost do it at the levels up to the point where the spell level bonus would stop. For example if you got +1 1st,2nd and 3rd level effect. Up to 5th level the PP gained are roughly the same benefit. Unfortunately the PP gain continues past 5th level which it wont for the spells.