"The best exhaust is no exhaust"
Unfortunately we can't live by that mantra, mostly because we like our ear drums intact.
If you're willing to spend some coin, depending on the diameter of the exhaust already (2.5" N/A and 3.0" for turbo) a high flow catalytic converter and a good high-flow muffler will do quite nicely to round out the exhaust system, resonators as required.
Anything in the exhaust system creates backpressure, the minimum requirement is a cat and a muffler, depending on where you live. If it is too loud still, then you can add on resonators as required... otherwise, go with the minimum.
Good high-flow cats will leech about 2-5hp off your engine... which is about the error margin of a dyno, so technically it's immesurable. Might as well save the environment a smidge, eh?
I don't know enough about that particular motor to help anymore. It sounds like you know what you're talking about, so I hope the above helps
