I think that shaving the heads will get a lil more compression out of it. I dont know what the pinion angle would have to do with it.
See the rear springs actually twist or flex when you launch. That right there actaully changes the angle on the rearend. The harder you hit the stock rear springs run the risk of actually bending the spring or breaking a mount to the spring , or even breaking the spring too depending on how old they are.