exhaust size

I would talk to the exhaust shop that is doing the work for you before bidding on those and see how they feel. They should be able to expand the 2.5" pipe to fit the 3" muffler.

Flowmasters arent the best flowing muffler out there and the internals of them will be the same regardless of what size the inlet/outlet is. I'm willing to bet you wont see any difference in low end torque between the 2.5" muffler and the 3" mufflers. If you were to run a 3" pipe you may loose some torque though.