The issue with being able to put the finger on the trigger is one that Safariland is aware of, and I think they're doing a better job designing the holsters to avoid it. They've put out an advisory basically saying that sometimes to have a light on a gun the opening might be big enough to allow someone to reach the trigger, and that agencies need to check duty holsters before approving them.
In terms of them doing a better job of designing holsters, I have a 6004 where I can reach the trigger with a gun in the holster, and a 6280 where I can't. Admitteldly, the 6004 is the model for the M3 light, while the 6280 is for the X200, but the size difference is fairly small between the lights. (Springfield operator w/x200)
Have you shot the M&P yet? Apparently, one of the large agencies in my area is going to them, although I would doubt they're going to be allowed to carry with mounted lights.
Hope this helps a little!
Dp