I heard...
That a turbo car, any exhaust restriction after the turbo (this is the major bottle neck) is a bad idea. The optimum set up would be to have about a foot of pipe after the turbo and that is it.
There are a few cars now that have turbo, then a bit of a backbox, and it exits infront of the front wheel. Save weight, less resistance. There was a Skyline like that at Japfest a couple years ago.
I'm no turbo expert, but thats what I heard/read somewhere.