A few years ago, I did a king size on my dsm using high-loft batt, because I wanted more of a comforter effect. It is no heavier than any other, and it is not stiff. I did a combination of quilting with the walking foot, and free motion. I did it with the 3-part technique from Ann Peterson's online class (put batting on the middle third, quilt it, then add batting to an outside third using batting tape, quilt that, and then repeat for the last third)
The only warning I would give is that if you use spray basting, get it right the first time. If you try to reposition the material after you've already set it on the batting, the batting tends to come apart.