Vinnie the clutch is not going to determine your top speed. Thats all going to depend on you engine and your gearing.
What those holes allow you to adjust is how soon or late in the RPM range the clutch engages. If you are a high traction surface a higher RPM engagement is good. If its loose and slippery then a lower RPM engagement will give smoother more controllable take off. If you were trying to get the truck to wheelie for example you would want a higher RPM engagement to snap the front wheels into the air. More bottom end will give you better acceleration out of turns as well because the engine will be spinning a lot faster when the clutch engages, which can also help you get over some jumps. Ive never tried the adjustable clutch shoes. Ive only used different weight springs to adjust the engagement. I hope that made sense.