Stay with the stock OEM belt. You will not see any increase over stock with different sizes. Would almost certainly result in poorer performance. A wider belt for example, will not increase the "gear" ratio. That is controlled by the size of your variator and rear pulley. I agree with Troutbum, your best improvement can be made by fine tuning your variator weights to keep you in that RPM sweet spot in relation to your weight and road conditions. Good luck!