2x +6 is huge though... and howler gets x2 -3 to counter it
and as to Drown Noble race, another idea that found its way into my mind was a BUR upgrade item (tiamat blood - bur set loot drop) that could only be used by a character with an open, secret or UR subrace and would then essentially enhance that character as if the subrace were of BUR quality
I would prefer the Protean race thats been sugested for a bur theurge race, then all Theurges would be very unique, and allows the person to choose their spell focus instead of the standard you set might float everyones boat
Protean would only be a rare drop on one of the bosses from the Elder Evil, so it may be a long time coming.
In general, I agree that +6/+6 for int/wis is a huge bonus, no matter what the penalties are, specially if you are still going to include the bur caster subby bonus feats (SF/GSF or SP/GSP)
+6/+4 to main/secondary would in fact be more than enough. Even +6/+2 to primary/secondary would be more than enough. Or maybe make two bur subbies, one for arcane and other for divine theurge, if that is easier.
Love the idea, hate the name. Chosen on Mystra are near immortal, uber spell casters with incredible special powers granted to them by Mystra (such as the "Silver Fire") Azuth (or Savras if you wanna go with the cannon that Azuth is dead), if one insist on keeping it as a "chosen" of a god. That would make a better choice since there is no preconceived ideas as to what a chosen on those two are. No, Magister is something unto itself and not a chosen. Sorry, my 2 cents being an old PnP player myself