I'd think MPG will be largely similar, perhaps a teensy bit less, to the tune of 1 to 3 MPG at the worst. It might decrease if you twist it more to hear the sound of the better pipe. In some cases MPG can even increase by changing the exhaust. Power/torque will go down slightly or move slightly in the rev range if you put the more restrictive standard exhaust back on. An aftermarket exhaust system will usually be more free flowing with increasing diametre piping towards the end can. This all helps power/torque and the helps with the bike revving easier. The GPR pipe is definitely one of the better designed systems, you'll likely feel a significant difference if you put the old exhaust back on. The difference with just putting a different end can on my old Wee was already very noticeable, it wasn't even a 3/4 system such as the GPR. I've gone and bought a GPR recently so that I can fit it once my bike has been put back together. If it's too loud for me (very unlikely) then I will do what Locky did and extend the baffle a bit. Extending the baffle will add an extra bit of restriction, but no where near the level that the stock exhaust would.
