Well top speed would be increased but with stock power I dont think it would be benificial unless you drive on the streets and need high speed. I think it would take too much power from the low end of a stock bike. Im not sure just guessing since I did mine after the big 3 mods.
I was looking for another gear on my stock raptor until I put a 15t on it and I highly recommend it as the 700 is geared very low and makes peak power at 6000 rpm. I've since done some mild mods but there are no drawbacks to going to a 15t on a stock machine unless you're crawling rocks! Top speed will be higher simply because you won't have to rev as far past the power peak and the bottom won't suffer a bit. The first three gears are much lower than on the 660 so raising your overall gearing matches the tranny gears perfectly! I run 15/36 and it's an improvement in all gears, at any speed! :thumbsup:
I highly recommend it. I've only got a PCIII, ProFlow intake and stock exhaust with a Phoenix spark arrestor, so maybe 44hp (guesstimate) compared to stock 40 (and I liked it even before the mods), and it's a great mod for little money. Go for it, it's dirt cheap to try it out for yourself and see how you like it!