Is your car a 997S or non-S? The results obtained with various exhaust components are different for each.

The 'S' benefits from cats most with a little extra from a well-designed header. Mufflers give very little except to change the sound. The 3.6 on the other hand has better headers as standard, picks up much less from aftermarket cats but does seem to benefit from free-flow mufflers and tips. IIRC, the standard 3.6 muffler is more restrictive than the standard 3.8 'S' design - they're different internally, I think.

So for an 'S', go for cats and headers first - I'd recommend AWE. For a non-S, replacing the mufflers is the way to go. I'd just have to bolt on a pair of Tubis with 'S' tips. They sound absolutely awesome at WOT but are very restrained at part throttle. They're also freer flowing than the standard, non-S muffler design so they should give you about 10-12BHP.

 

AWE 200 cell sport cats for MY05-MY08 and models 997/997S - $1,300 shipped to 48 states. Slightly used, great sound.