You'd be right about driveability if the Mustang was designed to do anything but go straight. You need to consider the car as a whole not just the engine. The first upgrade any Mustang needs is a fully independent rear suspension. Once he does that, smaller quick spooling turbos will give him a well built car. Otherwise if he is keeping the stock suspension then huge turbos won't make the car any less streetable than it already is. Hell removing the brakes and steering won't make the Mustang any less enjoyable in my opinion.
"The 300ZX Turbo is a dance; it's a song; it's rolling, roaring automotive art. There is no color that doesn't suit it. There is no mood-lifting chemical substance it can't replace." - Automobile Magazine
