I'm not Mike but here's my 2cents:
I've used a JRB No Sniveller in a bivy with NO problems. I'm very happy with it, even though I move around a lot, and enjoy sleeping on my side.
Try opening a bag inside a bivy one night as a test - and this will get you started in the right direction. You'll know if the quilt's shape (which is different from the bag's) is better or worse for comfort than your test night.
If you like it, then buy one! I can tell you that for overall comfort and "body-conformity" I much prefer my quilt to an open bag spread on top of me.