We all know the benefits of properly-chosen and installed aftermarket exhaust & intake, as "RaWarrior" mentioned. What the actual numbers are on a dyno are subject to debate, as different manufacturers' dynos give different readings, and they all use 'correction factors' in an attempt to stabilize things for temperature, relative humidity, & atmospheric pressure (elevation).
I think it was "Sketchy" who pointed-out to me that the CN bikes do not have performance penalties, and "Prez" has pointed-out here it is the E.U. bikes that had hardware differences resulting in lower h.p.
I have heard that one way to recoup your lost h.p. is a simple bit of maintenance (just fix this):