BCSAutomotive do a downpipe and 100 cell sports cat the best quality/design (thicker flanges, larger flexi, 5" cat instead of 4") i have seen. Makes a big improvement in response and doesn't bring the engine light on, as the cat is larger than all the others on the market.
Friend at work with Mountune260 and Mountune/Miltek cat back (70mm not 76mm/3" !!!) had them put on his and made a big improvement.
Got them on mine, soon be getting there cat back system also .