Progressing a PrC past its max level makes Metamind a little more viable, pre-Epic. Not much, but any little bit does help that craptastic PrC.
But you're losing even more MLs. DOES NOT COMPUTE.
Yeah. Nevermind that one. Don't know what I was thinking...other than wishfully.
How about paragon classes?
The only racial Paragon class with a feature that has a noted progression is Gnome Paragon, which would get a bigger boost to Illusion Aptitude. As for how optimized it'd be...hard to say without statting out a Bard/Gnome Paragon/Sublime Chord/Shadowcraft Mage/Legacy Champion.
By my reading, Legacy Champion could wreak havoc with any of the Martial Adept classes and PrCs. You get +8 Initiator levels, plus 1/2 of the levels of Legacy Champion, for a total of +13 Initiator Levels for ten class levels. Please correct me if I'm wrong.
You could maybe pull of some shenanigans by combining Legacy Champion with Uncanny Trickster and using the +1 level of class features from one PrC to gain the class features of the other PrC AND the class features of another class. Worthwhile? I dunno.
Two Psionic PrCs that would really benefit from Legacy Champion are the Meditant and the Crystal Master from Mind's Eye. Meditant would get another 8 activations per day, and could spend those to unlock more of the Intense Psychic Meditation options. Unless the text of Crystal Master is read to say that the class has a hard cap of 5 embedded gems, then you're looking at an additional 4 gems to add to your character's body. Plus those gems that grant powers which increase when more gems are embedded would further increase in power.
The Constructor wouldn't benefit as much, but it would still get two more levels of Boost Construct, so two more menu choices for those ACs.
The benefit to Dweomerkeeper is obvious: more uses of Supernatural Spell per day. Additional selections for Mantle of Spells is gravy.
If the Incantatrix's Instant Metamagic feature starts progressing at level 7 with an additional use every two levels thereafter, slapping on Legacy Champion could net ya 4 more Instant Metamagic applications per day.
I'm just brainstorming, here. I really don't know anything about the Legacy Champion besides its feature of progressing other class features. Don't know if any of the above would be good choices in practice, much less optimal ones.