Originally Posted by twinrider
I used to live down the road from a shop with a dyno so I dynoed my bikes (1998 Bandit 1200S, 1998 ZRX1100, 1999 VFR800, 2002 FZ1) before and after installing slip-ons/ full exhausts. The dyno runs showed good gains without making other changes. Yes, I got even better improvements when I changed the fueling and air filter but the initial gains were still pretty good. Those are facts...
It's critical to do before and after runs on the same dyno, same day. If you didn't do that, you won't have an accurate baseline to judge change.
Btw, had Akra slipons on my 2005 TDM900. Moved so I never dynoed it at that shop but they definitely woke the bike up. Switching back to stock for inspection and the bike felt strangled, especially at the top end. Akra didn't become an incredibly successful premium exhaust maker by publishing bogus dyno data...
All those bikes you mentioned that improved with a new exhaust are at least 10 years old or more. Maybe back then you'd see an improvement but today's stock exhausts are pretty damn good at flowing exhaust gas, they have to be. Only reason to change an exhaust IMHO is for sound or a small weight saving, but hey lets face it, most of us on here could produce a lot bigger weight savings going to the gym, me included.