Any weight that you save that the motor has to turn tires, axle, rims, hubs, chains etc. I think imo would be a better benefit for horespower & torque like Jesse 1004 said. Lengthening the swing arm has lesser benefit yes it probably get the tires spining better starting of in 2nd gear. Is your gearing correct for what you want to do? Is your jetting spot on? Pilot, Your L1, Diameter of needle, that comes into to play comeing out the hole. If you think it's correct. I'd get the axle before the swing-arm. This is my opinion