doesnt matter if you cut them or not, when you gear down the sled has less load and will pull the clutching easier, as the steeper gear loads the motor less making the work easier for the engine. This way you will also find out that the steeper the gearing you run the more it will over rev and will need more primary weight to be kept at the correct rpm.
Just cutting the clutchs will enable you to go further into overdrive, which isnt always faster as its more inefficent, your bending the belt in a tight radius in the secondary clutch then. Youll need to test and see which set up will give you the best mph, staying closer to 1;1 drive or getting into overdrive.