Regarding power curves.
The majority of the shaping of the power curve is going to come from the engine pipes, not the exhaust itself. On an NA car, the exhaust plays more of a part, becuase you are talking abuot pulses etc. On a turbo car, the impeller gets rid of the pulses and turns them into spinning air.
The best bet with a turbo car, is to get the biggest from the turbo back as possible. and especially right at the outlet of the turbo, because pressure differential between the sides of the impeller wheel plays a big part in boost response.
On a turbo car, the less back pressure, the better.
I'm not sure about the exhaust versus power equations, but I don't think I will ever be needing a bigger exhaust then the one I have (Fujistubo, Power Getter). I think its 3" or a little bit over.
The ultimate front/dump would be the CES Racing one which is split for about 40cm until just before the cat
Then a BATBML big arse cat
Then a full exhaust then there on.
Couldn't go too far wrong with most exhausts as they are all pretty much straight through anyway.
Are you coming tonight?? You could have a good look and listen to mine if you want.
BASS OUT.