i had 500rwhp in my cobra with a $700 blower port, $250 boost a pump, $500 in tuning/dyno time, $150 for my intake, $400 catback, and $400 for miscellaneous (pulley, blower belt, plugs, etc). why would you spend $20k in upgrades to a GT, when you'd be at or beyond the price of the GT500, which is a much, much better starting platform.
for the vette, its really not that difficult to get 500rwhp, and get the car's weight down around 3,000 lbs. less weight, means less stress on all drivetrain components