b/c it has to bolt-on to the stock midpipe.
Same with the RB exhaust - 3" pipe to the Y for the mufflers, but has 2.5" flange, which I'm seriously considering to eliminate.

 

NOW i see, I thought that was the end going to the mufflers, not the mid. Now it makes sense.
So it is a 3" exhaust, not much they could do about the neck down to make it fit!

Yes, eliminating it and getting a full 3" would be cool! Does the RP supercat/midpipe have 3" or 2.5?

 

Should be 2.5 b/c it fits all stock and aftermarket catbacks.
The only one full 3" is TurboXS. Comes with midpipe and catback.

 

Won't even matter. You have to do a lot more than eliminate one side of the c-pipe, you would have to open up the front of the muffler(s) to 3.0", the internals, and the outlet to 3." Right now the problem with the exhaust is it is all 2.5." Perfect for an NA car. Even then I kind've doubt the 30-40 whp gain when going to the 3" cat back. When SP Engineering disconnected the entire cat back, they got about an extra .05 BAR (less than 1 psi) gain. Told me that a straight midpipe might definitely helps, vs. hi-flow cat.