Yeah, I don't shoot f/1.4 much in studio, it's more useful outdoors where there is a background to blow away. Even still, to get the full benefits of resolution, you're talking about a tripod shot with a still subject. A manually-focused, handheld f/1.4 shot is totally possible, just expect a low keeper rate. I've shot handheld shots at f/1.2 with the 6D + 85L of still subjects, and even then I take a lot of shots. And slight wavering back and forth can lose critical focus.
I also have the precision matte focus screen installed in my 6D for just this purpose. The 5DIII and 5DSR can nail autofocus with the 85L using the precision spot AF, but my experience with the 6D is that its AF--even the center point--is too large and not precise enough to get focus right with very fast primes. My AF keeper rate with the 6D at f/1.2 is probably around 25%. And that's with the center AF point and no recomposing. With the peripheral AF points... forget about it. 5DSR on the other hand manages as near a 100% keeper rate as makes no difference, the benefit of the better AF system. In the studio, stopped down past f/4, both cameras AF well.