When we first tested the difference in the 3 to 2.5" vs full 3". We were still running stock intercooler pipes. In that configuration the 2.5" rear or the 3 inch rear made exactly the same low end power and the 3 inch had only slight gains at the high RPM area. There was never a time the smaller pipe showed more gains on the cat back. Now that we have larger inter-cooler pipes the 3 inch do make more overall power.
I was led down the wrong path many years ago when testing open downpipes. A 3 inch open downpipe vs 2.5" open downpipe showed low end gains on the smaller downpipe. The downpipe is a critical area.
