Regarding backpressure (when using a cat), see:

http://www.airpowersystems.com.au/us...wrx-hoe-r2.htm

There is no difference in backpressure noted between a 2.5" (R2) and 3" (R1) diff-back system, up to 525HP. Granted, this is just testing the APS system, and assumes that they used a thorough and accurate methodology for logging that data.

And no, I am not on commission from Michael South