Originally Posted by msgofast

You want it run it down from 3" at the diff flange with a taper to 2.5", keep it 2.5" to the end ( can run bigger tips), muffler needs to be 2.5" as well, also leading into the muffler 3" pipes used to like 2 x 90 degs bends for torque, with the 2.5" arangement it likes 2 x 45 degs bends , 2 x 90degs bends is still good, and there is very little difference in it, nothing you can pick other than being on a dyno.

I did an 02 wrx last week that started with 209kw AThubs was running a VF29@1.2bar, up pipe, STI TMIC, silicon inlet, ECUTEK with MSR/Hitech system with twin tip rear muffler which has 3" into it, twin pipes are 2" and = 2.75" area, fitted a 2.5" rear section and gained just over 15kw at 4000rpm, 2-3kw at 6000rpm.

Pressure tests just after the turbo were the same readings with both set ups, what you are doing is keeping up the gas speed as the gas cools, you loose 60% of exhaust temps within 300mm of the system.
STD exhaust front and mid sections are 60mm, or 2" 3/8", rear section step down at the flange also to 50mm odd.