Well that is not correct. STiffer spring does not mean tighter belt. But you need mor force to move/variate. But if you compensate with more weights in front you get more force to variate.
The only problem I have had with to tight belt, is that it not variate all the way, and you loosing top speed. But not power.
Another thing, stiffer spring in rear means more grip on the belt on the rear pulley, this means that you actually can losen the belt more without loosing the grip. (edited)