For the track, you are supposed to gear for the front straight, aiming for max revs in the highest gear attainable (some straights aren't very long).
As for the RC, I just went to 15/43 and it works very well. Take-offs are very easy, but I still have to do some clutch slippage to maneuver around in parking lots. Acceleration is excellent. Engine braking is increased, but I use it to my advantage to set the bike for corners. RPMs on the highway at around 72-75 MPH is 4500, which is right in the sweet spot for the engine. And, the stock-length chain will accomodate the 15/43.
That being said, the most popular combination is 15/42. I went a little higher as my goal was to have the top end of 1st gear be 60 MPH.
Eric