Well I just got some cobra speedsters and FI2000 installed and jeez are they loud. It sounds great, but very loud. I may have to get some new baffles to quieten it down a bit.
There is a definite improvement in the low end, more torque and you can do lower speeds in higher gears, and higher speeds in lower gears and don't have to grab that clutch so quickly when slowing down.
There is however a loss of power in the high end at motorway speeds, once you hit 70mph it definitely takes longer to get up speed. With the stock pipes I had this baby at 100mph with still more to go, after installing the cobras I was at full throttle at 100mph. This is probably no big deal really as I am not a speed demon and I rarely exceed 80mph anyway.
i am going to tweak the FI2000 a bit and see if things change.
I also did the airbox mod and noticed no difference what so ever.