Cross Sectional Area for a 2.5" pipe is 4.91 X 2 pipes = 9.82"
Cross Sectional Area for a 3" pipe is 7.07"
Cross Sectional Area for a 4" pipe is 12.57"

All of the above will flow more then enough for these 3.2L motors.

Dual exhaust is about pulse balancing and sound.

There are countless 500RWHP Supras and Camaros running a single 3" exhaust and the same amount running dual exhaust (no supra on that one).

For a street car it's a personal preference. For a race car it really depends on what your building it for.

Anthony
TVT Design