"I had a 2007 Kawasaki Vulcan 900 belt drive. I replaced standard pulleys for improved top speed and less vibrations but lost some noticeable torque. I then installed a K&N filter and frankly I could not believe the difference it made. Revved much harder, sounded better and improved my acceleration noticeably and regained any lost torque. I don't know if petrol consumption improved because I tended to go harder though I didn't notice any drop in MPG. Now got a Triumph Tiger 955i with K&N and it goes really well.
I'd recommend this to any bike owner. I'm now thinking of putting one in my 4x4 cause its heavy on fuel. FWIW."