I'm running 2 1/4" plumbing, but something to consider on the seemingly small 1/4" increase in the pipe diameter is the fairly large increase in area this provides. (piR**2) For 2.25 pipe thats about 4 sq in. for 2.5" pipe it's about 4.9 sq in. Thats almost a 25% increase in area from about a 10% increase in diameter. That extra area (and resulting volume) will somewhat slow down the evacuation of exhaust gases, but I don't know if it's "too much". I'm sure there has to some big pipe SVT'rs out there that have some real experience with this.
