You're missing the point. Going fast, and having a car you can drink tea in at 155mph are two different things. Porsches/BMWs/Audis/Mercs may be overpriced, slower than Z06's/CTSV's/ etc per/$, but they deliver something that Chevy and Cadillac can never deliver. Name, pedigree, fit and finish and the refinement of something that is designed to run all day at 100mph+.
Someone who might buy a CTS-V would never cross-shop an S6, E55, or M5 - and people who are die-hard German buyers would never even consider stepping foot in a Cadillac dealership, no matter how cheap the car is.
Sure, you could build a budget fast car - but what it makes up for in speed and stopping - it loses in comfort, stability and reliability. Drive a C6 Corvette, and then drive and S5, and you'll know what I mean. The Germans know how to do it.