I have an SDN6 that sounds good but degrades performance on pretty much all the rifles --others say they haven't had that problem, but I do have it on several rifles in different calibers. They all tend to open up, most about 1-1.5MOA. I chalk it up to the QD mounts though --the one I ground for a tighter fit, it opens up much, much less, .67. The rest wiggle and/or rotate freely to some degree. I find the groups tend to be a tad bit lower too, particularly on the 5.56 M4, hardly at all on the heavy 6.5G barrel.
I'll be sticking with thread ons from now on though. Most of those guys say it helps or does nothing negative.