[TurboKraft]

4" is not necessary.
3" is plenty and will support lots of power.

We just used magnaflow's largest 3" muffler with a GT35R on a turbocharged 964 and pulled a repeatable 520rwhp/550rwtq. Backpressure is not an issue. (And the car was funny quiet with that huge muffler.)

2.5" flows better than many suggest, too. Case in point:
- 2.8L 951 making 400rwhp on a 2.5" system, gained <24hp when switching to a 3" system -- less than a 7% gain when others were saying this car was choking on the 2.5L
- recent 3.8L Twin Turbo -- 657awhp through 2.5" tubing & mufflers after the turbos

When the system uses all mandrel bends and free-flow straight-through style mufflers (Magnaflow, Borla XR1) then you reduce backpressure, boost kicks in sooner, you get the cool whistling sound through the exhaust, etc.