Last year I spent the night down to 4*F with a Military MSS, both bags inside the bivy cover, and a 3/4 length 1" Thermarest. I had on long johns and a micro fleec pull over. I was good to go, with minor cold shoulders a couple times. I just roll over to warm that side.
So putting a pad with a quilt in my opinion will definitely get you down further. But then again I have used a pad for the last 3 years. YMMV
I just bought a set of 20*F Jacks R Better quilts and gonna go out this weekend to test them out.
Yes adding a quilt will extend the range of the pad(s). However, just adding an additional pad sideways to cover ones shoulders, or a pad extender will do the same thing and also doesn't have the downside of the heat stealing wind.
Insultex works pretty well. Have used a 16" x 5 ft piece along with a DIY underquilt made from one of my grandson's old synthetic sleeping bags and stayed warm into the thirties in rain/wind.