2 1/2 inch pipe should probably be the max size. People don't take into account the many areas to consider when building an exhaust system. 3" duals is not even rec. until you reach 500+ HP. Flowmaster and Magnaflow both make it a point. You have to consider the exiting diameter of the pipe, and the external atmospheric pressure it must overcome, 21 psi. With smaller pipe what you loose in volume you gain in velocity. The bigger the pipe the less velocity, plus it makes it engine to work harder to scavenge the cylinders.
Take into account a motorcycle. I had a 750 that had 1.5" duals on it, with 3" mufflers on them. I wanted it louder, so I removed the baffles, afterwards I lost almost all low-end torque. Same thing on a different scale.
The newer chevy trucks do have 2 3/4" dual pipes going into the muffler, but if you cut that muffler off and apart. It reduces it down to 2 1/4" into the muffler with a single 2 1/4" outlet, bumped up to a 3.5" tail. It maintained the volume, and gained the velocity like a venturi in a carb. BTW the ones that look like a dual outlet are a single outlet on those trucks. The pipe closest to the driveshaft is just an inlet to a resonator chamber on the back side of the muffler.
