mitsubishi managed to put a tt 3.0L in there car. they even bolted it to an all wheel-drive tranny. look i wasn't trying to start a war, all i'm saying is that if a company will mass-produce a car with 320hp and 315tq stock out of a little motor back in 1992 then surly we can improve on there design.....
But have you seen the size of the 3000gt engine bay? It's huge... you could probably fit a triton v10 in there . It's not a matter of improving a design, it's a matter of space and need. If there is a single turbo design that spools quickly and is capable of 300hp+, why would want a twin turbo? More heat, weight, complexity, etc., etc...