Well, the idea of good backpressure applies to a certain range of RPM, or well exhaust flows to be specific. Exhaust is a high pressure gas, so when it is released it creates a pressure wave. The theory is you want that wave to move at a specific speed so that the trough of this pressure wave, or the lowest pressure possible, is where the exhaust valve is located. Because a high pressure tries to equalize out low pressure, the high pressure exhaust that is now exitted is thus pulled towards the lower pressure, so instead of the piston having to push the exhaust out, the forces of nature do it for the engine, thus putting more power to the ground. It's actually a highly provable theory, just none of us people with V6's have enough money to sit there to gain flow charts of each car. Now once you start making more exhaust (increased air and increased fuel in each cylinder making more hp), then you need a bigger exhaust pipe to keep that range of effective "Scavenger Effect" inside the "Power Band" which is somewhere between 3200 and redline (5500) for our cars.
Now for why you feel some effect with 2.5" is because it flows well and the piston does less work when it does have to push it out, but the effective scavenger effect occurs much higher so in certain ranges, each size will have it's benifits.