If GM is doing "reasearch" on a C5/C6 with a bigger engine, they're doing it with the C5R "motorsports" block which is part of the Gen III engine family.
Typically, this engine is used in a 7.0L (427 cuin.) configuration.
The biggest problem getting a 427 to production will be CAFE and emissions with CAFE being the tough one.
If GM goes to production with a 427 in some limited production Corvette, look for some kind of cylinder de-activation system to get the required fuel economy.
Other choices for more power...lots more power, might be a supercharged 5.7. 450hp with a Roots or screw-type blower would be a snap for the Gen III V8.