I've got a 2.25" catless testpipe. I put it on once for a few days. And it came off immediately due to the sound.
I have borla headers with a 2.25" unresonated midpipe welded to a WRX axleback. Stock cats and I love the sound. However when I put the testpipe on it dramatically increased the sound and the sound became very tinny and raspy. So much that my GF would not ride in the car.
Now you will definitely feel a gain when accelerating above 3500-4000rpms. I didn't notice any low end loss. But I've got to figure out how to muffle the sound before I dare put it back on.