I've seen a factory plastic rail shoot loose, but it was not pulling the threads out of the receiver. The Beretta screws were actually failing and the heads snapping off them. I addressed this earlier in the thread, and it came up again a month or two ago.
I'm thinking that by using the aluminum Gen II rail so that flexing of the rail doesn't concentrate stresses unevenly, and suitably upgraded screws, this should be well alleviated, as similar setups don't seem to be a problem on rifles with much heavier scopes mounted. An aluminum rail and properly manufactured screws are also integral parts of the CROM solution.
Does your experience say otherwise?