The thing about your mirror is all the weight is pulling perpendicular to the wall anchor. That's where it's strongest. When using a wall anchor on a speaker mount, much of the force (at least on the top-most screw) will be acting to try to pull the anchor straight out from the wall. The Axiom FMB mounts attach to the wall at two points.
Just two points vertically aligned? I still wouldn't be worried about them. I would think something that horz, would have something to both sides on top then a bottom mount just for "bounce"
