My trackbike used to have the secondary air system functioning, doesn't any more, didn't sound **** then, still doesn't.
During development the manufacturers spend ages working on the fuelling based on the specific back pressure generated by the standard exhaust, replacing the silencer with one giving less back pressure will obviously have an effect. Even when just fitting an end can the fuelling and thus performance can be improved by proper setting up using a dynojet kit/power commander. The effects of set-up are even more pronounced if using a less restrictive air filter and a full race exhaust system.
I've ridden my mate's ThunderCat and the benefits of it being set up correctly are very apparent, good response and clean fuelling throughout the rev range. For a not-so-new and relatively lardy 600 it goes surprisingly well.
