Horsepower shouldnt effect the k-members integrity. All aftermakret k-members that I have seen have been up to PAR. Im using the UPR peice, no complaints with 615 flywheel horsepower. Id be willing to say that an aftermarket k-member is capable of supporting or withstanding more abuse then a stock peice. most companies use chromoly steel, which is pretty damn strong, compared to stamped steel that the stock peice is made out of.
Besides weight saving, aftermakret k-members offer-
-Use of coil over suspension
-Increasred oil pan clearance
-Increased header clearanced
-Easy access to starter -
-Looks AWESOME!
When i did my swap, the actual k-member weighed 13lbs.... .the stock peice was around 45lbs...
Considering that just about every mustang that is a dedicated track car has one underneath it. Mustangs that are down in the 7s have them so I think there pretty tough.