You are comparing a sorcerer with fully scaled up damage against a psion removed of every advantage they have (most importantly the flexibility (both in their known powers and their free application of PP), really).
A 10th level fighter can do two swings per round (one damage die each), a day has a lot of rounds...
Still a sorcerer beats the fighter in raw damage output, since they can unleash a higher potiential in a shorter amount of time (and for this reason only).
The psion is the next step up. They can unleash even more potential in a short amount of time.
But they do not have to, just like the sorcerer can use lower level spells (which you said yourself are best used for spells which do not scale (and thus cap out), like Fly), they can achieve things with manifestations, which are not at their maximum. Their are plenty powers, which do not even need to be (or can be) augmented and still have their full effect. Did I consider these? Nope, and they will only further increase the discrepancy between the classes. By doing this a psion has a very good endurance, too.
Yes, indeed. I do not consider it fair to include all advantages of scaling and to remove all the advantages of augmentation. This is exactly what your example did. It's pointless and meaningless.
At epic levels?
And the point where the Mystic Theurge actually becomes very good is the point where they reach 9th level spells, since then they have something in addition to what other casters have available. That is for one reason mainly, because there are no 10th level spells. Prior to that they suffer from their lack in potency throughout their whole carreer (tho they are still useful characters, just not in the power department). But this is something unique for the Mystic Theurge, it doesn't apply here.
15th level sorcerers (to pick one of the examples, the general idea is true for all, tho) do not gain 6th level spells in addition to psions, they do not even gain 5th level or 4th level spells, what they have is an amount of 1st through 3rd level spells (all of which are already far beyond their cap), which they have after trading their 4th-7th level spells one for one against fully augmented 8th level powers
each. And
each of those 4th-7th level spells had a weaker effect in the first place.
And this is not even damage dice only, it's just looking at the level of the spells/powers for a general effect (full scaling versus full augmentation), which certainly is very compareable.
Bye
Thanee