How expensive are the twins for the Ford GT?

$7,500 for an APS kit on the C6Z and it'll make 700rwhp. +250rwhp over stock. Not to mention the bank you save off the initial purchase price. Another couple grand for forged internals (rods/pistons/valvetrain) and you're looking at 1000hp.

I've seen some pretty nasty TT Ford GTs and Vipers too, though.

Who knows. Hard to say what car is better without actually having them built and seeing them run.