I was thinking about doing the same thing. Out of the 3 cats on the car, the 3rd one downstream probably affects performance the least. The best one I've seen so far that replaces the 2nd cat is the bosal twin dump model.
I am not positive but i think if you want to go with one cat i would leave the last one, removing the cat on the uppipe seems to be a very good starter mod. The cat between the exhaust port disrupts crucial flow to the turbine. I would think that quicker spool (getting air to the turbine) not max boost (max flow after system is at equilibrium) is the limiting factor, except for very high boost situations(over simplified).
Any good uppipe and downpipe will do, seems mfg market the third cat replacement pipe as a race pipe or at least turboXS does.
TurboXS replaces the third cat with a high flow cat. They do offer a racing pipe as well though.
Scoobysport's exhaust peices all replace their factory couterparts. You can mix and match them at will. So you could get their down pipe and leave the third cat in. You could get their cat back and still have the third cat. You could buy their front center section and remove/replace the third cat as you see fit (track days, emission testing days ).
when i asked scott at suby specialties this same question today, he said go ahead and get 4 inches even if you dont care about sound. on a turbo car the bigger it is the more power you will get, backpressure isnt really needed
Which brands of 3 inch diameter downpipes will bolt right up to the Turbo XS test pipe back exhaust system? My friend's car has the huge 4 inch TXS downpipe on it, but he wants to change it to a 3 inch.
Yah. Turbochargers are by themselves a huge restriction of the exhaust system. While to some extent, gigantic I.D. exhausts will actally diminish torque on n.a. cars due to decreased exhaust gas velocity/extraction, This isn't the case with tubocharged vehicles, our wrx's included.