is it possible that if you do not have the correct belt tension the engine will idle higher

No but if your belt is dragging your idle will be lower.Sled should idle the same with the belt removed as it does with it on.I don't see any way it could idle higher unless you have the idle set too high and are using the belt set too tight to drag the idle down to normal.This usually results in a sled that will creep away on its own when idling.