12bolt is 8.875" ring gear, 10 bolt is 8.5"
The 12 bolt is NOT to be confused with the passenger car 12bolt, they are not the same. The Truck rear has a much smaller pinion. It is smaller than the 10 bolt.
As for how much power that isn't quite an easy answer. If you are drag racing it on slicks, eventually it will break. For street tires you will spin first...