It says right in the Feat,
"Saving throws and opposed rolls (...) are not affected."
Maximize Power maximizes random, numeric EFFECTS, like it says in the first sentence. Save DC isn't an effect of a power, it's a consequence of the power's level.
Now, there was also a debate a while back over whether a Maximized Astral Construct would get max HP. They asked the devs for this, and the answer was no. Basically, the only things that get maximized are the numbers written in the power's description itself. Anything that requires searching in other sections of the book wouldn't be affected.
Considering how lousy the psionic damage spells are, it's not a very good Feat. If you insist on improving your damage spells, though, use Fortify Power (it's like Empower Spell, but it's +1/4 damage for +1 level)