I agree with the points made by Keitho above.
I am VERY happy with my MSS two bag system with a gortex bivy. My grandson @ 6' 1" is already longer than I, and doesn't like the confines of a mummy. I was trying to guide him to a Big Agnus "Lost Ranger" bag. It probably doesn't have the girth you are looking for though. The concept of the bag is like what Keitho is talking about. It has no insulation on the bottom, which makes it more packable, and lighter. One relies upon a sleeping pad for ground insulation. The insulation of the bag is all on the top and sides. So it is like a zip up, wrap-around quilt. Hammock campers have gone with a top quilt and under quilt for years, and many tout the system.
Different sleeping pads have different R values, and have varying comfort levels for back sleepers vs side sleepers. To my surprise, I found that my Klymit V is more comfortable than my self inflating Therm-a-rest pro. I never particularly liked air mattresses, but I found that if I filled it up, then lied down on it and deflated it until I was comfortable, even side lying, my hips could still be off of the ground. I'm only about 180 lbs, so that model may not work for you. I'm sure the concept will be the same.
All in all, I'd go with Keitho's recommendations, and get a custom quilt made to my specs.
OK... as I went upstairs to get myself another cup of coffee, I had a thought.
I considered, that I might want to tear open the gortex bivy of the MSS and make it a water proof breathable top cover for the quilt. Then I thought.... why not cut open the back of the patrol bag, and/or the black bag, and insert some extra material to make it wider? This will allow you to continue to have the three bag flexibility, temperature range, of the MSS.
The Big Agnus is designed with a pocket for the sleeping pad. The last time out, I placed my Klymit between the gortex bivy and the black bag. It worked great!