That's what I'd be worried about with mechs at that kind of speed. Even one blade popping open on launch will throw it way off course. On the ones that use a rubber band to hold them closed, you might have to double band them. I know that Excalibur crossbows markets a head called the X-Act that uses internal clips to hold the blades closed. It's supposed to work on their high performance crossbows at 355 fps. I've never looked at 'em myself, but they might be worth a try if you decide to go that route.

If you do what you're supposed to and manually check each blade on each head to make sure they all move freely, there is no reason a mech won't open when it hits the animal. My problem with them is I've had to clean up and remove burrs and trash from at least one head in every package I ever bought just to make sure they all worked. I strongly suspect that is why there are so many stories about mechanical heads not opening. Boils down to poor quality control on part of the manufacturer and plain old fashioned operator error on part of the hunter.

All that said, I don't blame you for wanting to stick with fixed blade heads if you can. Problem is, to get the best possible broadhead flight and accuracy at that speed, you are going to need the highest quality arrows you can get - straightest and highest consistency in spine and weight - which also means expensive. And you are going to have to be downright anal about making sure there is absolutely no tip wobble when you spin them.