For as cheap as the R6 or R1 throttle tube w/grip, this is the best way to go.
The R1 will go on with no issues at all. The R6 may need some dremeling in some areas and some adjustments in the cable as well. I have the R1 mod as I have done this mod on all my bikes. You will get a little less than 1/8" difference where it stops. The R6 I believe double that. I think the R6 may be best for the little Ninja especially for racing but maintaining a steady cruise will be more difficult. I may actually order one up since they are even cheaper than the R1.
Stock
R1