The failure of Torx screws can be attributed to a lot of things. The sizes are very close and are easily mistakeable, if you use too small a size it'll round off the fitting and ya got a problem, some of the Torx tools are suspect, they break or strip and ya got a problem, the ones that open like a jackknife, wobble, and don't let you get enough force to turn the screw so they strip the hole and ya got a problem. And sometimes you use the wrong Lok Tight (Red Death) and they won't come out.
Here's a few tips,
Keep trying sizes until one is too big and then go down a size. Get yourself some GOOD tools. Snap-on, MAC or Sears Craftsman, not Harbor Freight or AutoZone and buy a socket set type of Torx, you can get more leverage and control.
Lastly, thank GOD HD uses a soft metal in those screws and watch out if your thinking about replaceing them with something "Better", Think about what kind of problems you'll have with harder screws..........I can see stripped screw hole threads, broken off gaulded screws and all kinds of trouble. Oh and use the Right Thread lock..