A downdraught and extractors and pipes will give you slightly more go although I think it will just "sound" better rather than "go" better(from experience). If you want a cheap easy upgrade I'd just drop an A14 (it will bolt straight in no mods at all) 1&3/4 or 2" pipes with extractors. See how you like it first and then think about a carbie . It's just too hard to get any real go out of an A12