personally i didn't rate the 2.5" exhaust on stock engines. mine went better with 2" even than the 2.25 one,(be different if jetting etc was being done on the carby on a dyno i suspect)
unless you like noise, the 2.5" makes a lot more noise, (i still reckon they sound better with a sports muffler on the stock exhaust size )
that's on a stock engine that only makes power to 4000rpm at best though (due to cam/valve springs)