I can't say I have ever seen a metal mount for the 509 helmets, or really heard of someone snapping one. I've used mine a bunch of times both sledding and quadding and haven't had an issue, other then the gopro getting rotated by the odd branch.
I've had mine for three years now, no problems. But to be honest, that's a crappy angle. Your better off getting a face mount or under your visor. My favorite is the Dango Designs. It stays put and clamp is handy to get b-roll and other shots. I've never had it pop off and I've faceplanted over the bars a couple times.