i did a lot of playing around with a dump pipe on my car, it was far from one of the bad ass cars but the principal is exactly the same. i went 2 tenths faster through the drump (crappy flowmaster muffler otherwise stock exhaust) i believe i picked up 1 mph and 60's were also faster. on the street it was very noticeable how much faster the turbo spooled, so even if you were to lose a little low end power (i'll get to that in a second) it would be imediatly compensated for by the faster spool. it works theres no question about it. a good flowing exhaust will be almost as good but i don't think you'll find anyone saying they ran better through exhaust.
as for backpressure, the turbo really plugs up the exhaust. i've heard people have seen around 16 psi in the headers of a turbo car. i'm sure thats not a set number by any means though. with a huge turbo i guess its possible that you wouldnt' get enough back pressure. but it would have to be so big you would never get a little v6 to spool it anyways so its none of your concern. the back pressure is there, if you don't believe it tap in a valve stem to the headers and hook up a pressure gauge and see for yourself. i'd put money on it being double that of most naturally asperated car through full exhaust.
if you like the sound then thats really the most important thing since it sounds to me like its a street car. personally i think my next tr will have a single 3" pipe from the downpipe back, i like the sound too but i don't like it coming from under the car.
