sorry for digging this thread up from the past, but it was better than starting a new thread on the same thing.
Just got a Barret slip-on, and fitted it today. It looks mean - black pipe with blue tip - and sounds freaking awesome! it slipped straight on, with no hassles. Except the spring, that thing is a bitch.
In theory, i should get a bit more bottom end power. Barret have power charts for various bikes with their exhaust vs. standard, but no '09 fe450. the exc450 (basically the same engine as the berg, right??) gets a few extra hp out of the slip on, and the curve is slightly higher at lower revs compared to the stock. So it should go alright. Will post a more detailed report on sunday when i get back from louee.
I fair reccomend these guys, as my housemate got the full system for his klx450 (adds about 5hp) for 800, as well as a lot of helpful advice (he's fairly skittish about buying stuff online), and even jet settings! The stuff only took 2 days to get here aswell, which isnt too bad.