See how there is a flat side and an angled side of each tooth?
The flat side is the drive side, the angled side is the coast side.
High pinion in the rear = driving forwards on the coast side... so the pinion is basically pushing the ring gear away from itself about as hard as it is trying to turn forward.
If you want a high pinion rear it needs to be BEEFY to handle the abuse... I would consider a HP D70 as minimum... and even then I'd consider pinion bearing replacements as a normal maintenance item every few years.