Stock pipe is 2".
One more half inch is not that much bigger,, especially if your muffler shop uses the standard tubing bender that crushes the pipes in the bends.
For example, your are using 2.5" tubing, the muffler shop guy bends it for the up & over the rear end,,,(or other joggles) if he does not do a good job, the inside pipe diameter could end up 2.25 inches or smaller.
Well ,,,with that,,, it is really only flowing like a 2.25 inch tubing.
Now going to two,,, 2.5 pipes that would be a big difference.
Dual exhaust, w/headers
Another point,,,,, the exhaust manifold will give you plenty of back pressure (does not flow well)
So I would not worry about running a 2.5 inch exhaust system.
