Best way to assume what you'll get out of changing sprocket sizes goes something like this, and it applies to all bikes:

First, going up or down one tooth on the front sprocket produces a similar affect to going down or up (respectively) two on the back. Keeping that in mind, while changing the front (drive) sprocket is easier it (obviously) has a greater overall effect upon the perceived performance of the bike. Additionally, I would strongly advise against going smaller than 14t on the front because you're reducing the radius of the chain's transition - this will cause greater wear on the chain not only because of the transition itself, but you'll be putting a greater load on a smaller amount of area at any given time. I tried a 14t on my Monster for a while, and I was shifting gears all over the place. Drove me nuts. (I still have the sprocket if anybody's interested. Made by Afam - has maybe 2k on it.)

Anyway, dropping a tooth on the front sprocket is similar (in seat of the pants feel) to adding two on the rear, is what I'm getting at; vice versa adding a tooth.

Now, let's say you decide to keep the front sprocket as is: if you go up +2t on the rear, you are effectively "lowering your gearing" - your final drive ratio might go from 15/38 to 15/40 for example. The seat-of-the-pants result is greater launch power and a lower top speed in each gear. So if you like to shift into 2nd at 30mph, you'll find yourself shifting into second - at the same rpm as before - at 26mph (just guess-timating numbers). Same obviously goes for top speed - if you liked to wring its neck at 130mph; you might only get 115 out of it. If you're not concerned with top speed and you only like beat people off the line, this is the way to go. I think this is what you're looking for ... ?

Conversely, if you go down two teeth in the rear, your launches are going to take more work (and a steadier clutch hand), but your top speed in each gear as well as overall will increase.

I've heard changing the tooth count in your rear sprocket results in something like a ~500 rpm difference per tooth, in the top speed in any gear, and the relationship is relative with consideration to speed: drop two teeth, your RPMs will be ~1k lower at the same speed compared to where they would be normally. Or, add two teeth, your RPMs will be ~1k higher at the same speed compared to the previous tooth count.

Confused yet? :neener:

I run a 15/40 setup and love it. it's just the right blend in my opinion.