well look at it this way... bigger the tire, more power you have to deliever to the ground but you cover more ground. Bigger the wheel, higher top end. So to fix that change your Spur Gear Clutch bell combo... I went to a 52T/13T. Plus I am running a WASP 26.... off road use is awsome. My buddy has the same set up but running on stock tires and a 52T/18T and i kill him.
Smaller the tire less power delieverd to the ground but more revolutions to catch up with the bigger tire so actually more power delievered... little low top end
if you go with the UFO's, just change your clutch bell to a 14T or 13T with a 52T spur gear.. RRP makes a metal spur gear in 52T. 52T is the best gear for either top end or low end..
torque power & combination is the key