First off, there is no best gearing. It's track, rider and pace dependant. I have been beat by 15/42 while on stock gearing and vice versa. For example; when I started riding mid-ohio on my r6, the stock gear felt perfect but once I got moving really fast, I went up one in the front only. At putnam 15/42 sucked, went back to stock and was it was t*ts. Play with it... there is no right or wrong answer here.
I feel a 13t would have you shifting like a mad man. Better go with GP shift if you go that route. If you have never been to the track, give stock a go first, take the gears with you, they are a piece of cake to change out during the lunch break. Don't forget your torque wrench.
