Dudes, the original question is so bogus! The only HORSEPOWER limit on a rear end would be the horsepower that would generate enough BTUs to cause the rear end to overheat. Example: NASCAR, where the Ford 9" they used required an oil cooler of its own. I don't know what they are doing nowadays.
What the OP should have been asking is how much TORQUE the rear end can tolerate. That's how axle manufacturers rate their axles, torque capacity.
Shock loads are also important and I think there's usually a factor for shock loads included in the calculations.
Why not use horsepower? Because an engine with rated horsepower at 5,000rpm will produce roughly 3 times the torque as an engine producing the same max hp at 15,000rpm.
R.
However that 15000rpm peak hp car would probably use 3 times as much gear ratio and at that point just as much torque would be trown at the axleshafts anyway and at that point you are back to squareone again.
