I used my former one with a 3" downmat, and with a thin JRB torso pad. Both mostly worked great in terms of fit and insulation, but I did not like being raised up in the RR while on the downmat, contrary to what I've heard about the reg BMBH. It put my knees right into the hard top of the sides, instead of into the soft body of the sides - when I turned onto my side. Laying on your back in the RR is a dream, but inevitably I would want to turn only my side for a bit, and it just didn't work out that well for me.
With a thinner pad, you're apt to get less of that. I would think you would still want something greater than the standard 20" width for pads.