The smaller the rear sprocket, the lower the engine speed at a given velocity. So, theoretically, if you drop a tooth on the rear sprocket you'll be going faster when your rev limiter kicks in, thus giving you a higher top speed. Since the engine speed is lower, you'll probably get slightly better fuel mileage. All this comes at the expense of acceleration, however. If you go up a tooth from stock, you'll have better acceleration off the line, but mileage and top speed may suffer.