I believe they are 2 1/2 inch dia. and the dual custom pipes are the same.

I don't believe larger pipes would make much difference but that is a tough question to answer without testing.

The improvements are theoretical and what I believe is improved higher RPM horsepower. It is a seat of your pants measurement but not too reliable for true measurement. The gas mileage is definitely improved over the factory specs. I proved that over 15,000 kilometer trip last year.

The sound is definitely better over factory but that was somewhat of a 'no brainer'.