We know how it goes: you've done your best to tune up your motor to bring it up to an appreciable performance level - then find that your speed isn't going any where cause the original transmission only helps with acceleration rather than bringing the power performance up where you really want it. Always at the limit, your motor works it's heart out for you, but..... which in turn means stress, wear and tear for crankcase, bearings and pistons.
A longer transmission gives you the chance to get the same speed - but at a lower rpm's for a healthier motor. Give yourself an extra margin and more performance without stressing out on inclines. Your acelleration lost can be made up with a variator readjustment. The rolls a little lighter, your clutch a little harder.
The sport transmissions from MALOSSI and POLINI are mainly straight geared, making the interlock area smaller and so reducing friction. Lower friction in your transmission means less heat and you're finally getting that badly needed power to your rear wheels.