If you're looking into a decent exhaust for your 6/3 plan about $2-2,500 minimum. It's basically from the turbo back, replacing the stock system, thus giving you more power, and if it's your desire, enhanced sound. The stock downpipe (turbo, into downpipe, into catalytic converter out...) is a large restrictive member in the MPS. The flow is poor, the design is large and considers emissions and the likes. Removal of this will give a increased exhaust gas flow and at less resistance due to the larger diameter of the downpipe unit itself.
The optional part now, is whether or not to get a cat-back system, flanged to the downpipe. In terms of "power" most of the gains you will be able to achieve are left at the downpipe. Although a smoother and larger diameter makes for less resistance and of course replacing the mufflers will give a sound to your vehicle. You will notice on the 6, the cat-back single pipe-run splits further down to enable entry to the twin mufflers. This split (or "Y" piece) is pretty much a curved "T". Having a "T"-like run makes for increased resistance as your exhaust gas attempts to exit via the mufflers. A high flow, less resistance "Y" piece (see photo's below) would achieve minimal gains but a smoother higher flowing system. Why not compliment the downpipe and complete the exhaust if your finances allow you?
Mufflers can be personal. Stainless, oval, cannon, round, high flow, sports, sound, loud are all the words that come to mind. If you intend getting a set of mufflers you want a note, but nothing too loud that will make the neighbours hate you and the cops like you. For me it was two stainless steel super-cat mufflers. They're oval by design and sit high up, thus offering next to no chance of hitting. The pipework neck from the muffler will/should comfortably exit the chrome exhaust tips sitting not to far out but rather inside about ~10mm
0₂ Sensor connection on DP
DP Flange to cat-back, can see one resonator and the other in the corner of the shot
Pipework connection into muffler
Rear setup
Exhaust Tip