Without question! I have yet to be able to shoot an expert score without the aid of a sling. FWIW, that's what I did with the XCR too - put a sling mount on the bottom rail.
The beautiful part is that you can position the sling attachment on the rail to act as a handstop too! That'll go FAR in not having to rebuild your position every few rounds. Literally.
FWIW, I went with the Midwest Industries..... actually, I bought two - the full swivel and the one that takes the Uncle Mikes QD swivels. Just wanted to decide which one I liked better. I've decided to go with the QD type for now, and see how that works on the COF coming up next month. It means cutting on a rail cover too, but for those few times I use the XCR in that way, who cares?
I bought the one from Falcon, but didn't much care for the way it secured to the rail. It went back. Not that I had problems with it, but saw it as a potential problem in the future.
It's for sure that the gripe won't be about the HK accuracy. Those things are GREAT in that respect. I took my current 91 out with a couple of friends last year (mine doesn't get much range time any more) and we were shooting at some larger backpack sized rocks around the 800 yard line. Once you figured out where to hold to get the round that far, the rifle wouldn't disappoint!