I don't think you can say at XXX hp, replace the driveshaft. Too many variables to consider.
But I do believe that you can state that if you are an aggressive driver or if you plan to visit the drag strip and make max effort runs with sticky tires (drag radials or slicks), then it is in your best interests to replace the driveshaft. A stock GT500 with good tires and suspension can break the stock driveshaft.
It may be more correct to ask" What is the minimum 60ft time that is considered safe for the stock driveshaft?"