2.5" inlet/2.25" outlet testpipe vs. modded 2.5" inlet/2.25" outlet ITR cat.
Will there be a big difference in power?

I'm running the T1R header, custom testpipe and Mugen exhaust and I want to put a cat back on since it's daily driven now and I don't want shitz from the popo.

 

the cat will dramatically reduce exhaust flow vs. the testpipe, but if its a DD i would probably put leave the cat on

 

Run a high-flow cat. instead. The difference in power between a test pipe and a high-flow cat. will be much less than between a tespipe and a modded ITR cat.