Stock diameter size is 2.25". I think that is inside diameter if I remember correctly.

A free-flow converter was added to each side in place of muffler. Originally, I did not have that (just tail pipes with tips) but I found that the sound was too loud, not mellow enough. I took out about a foot long of pipe on each side and replaced with the converters. I emphasize free-flow because that is what they are. You can see right through from one end to other. It has some muffling or catalytic material that acts as a catalytic converter but it sure made a difference in mellowing the exhaust sound.

I have no idea about the specs on torque or horsepower increase but I can honestly say that there is an improvement especially in the top end (2nd, 3rd, 4th gears). I wish I was in a position that I could have dyno'd the car first and then after with the change but that didn't happen.

I now wish that the 1st gear was not so low because the torque/revs come on so fast that you need to go into 2nd gear in a flash. Other than that, I can't help you much further.
I don't know if others who have 6 spd manual feel this way, but I find myself starting out in 2nd a lot (not always but a lot).