I use the high temp silicone stuff on mine and haven't had an issue but one of the things I do is to make sure I give the exhaust bolts a tweak right after the first run when the muffs are re-installed with the engine still warm, you usually get a bit more torque on 'em.

One other observation from watching a friend fight this battle with a ZDZ-80RV; once it happens you get about one more chance to get 'em tight or the threads inside the exhaust port get worn from the vibration. They may not feel stripped but if you play around with the one that's having an issue vs. the ones that don't you can feel the difference.

IMHO, once you get that extra slop/space in the thread contact area there isn't anything that's gonna stop 'em from coming out again except safety wire and that's not an option on a number of muffler installs. My buddy had to put heli coils in his ZDZ to fix the problem and it did work.