SB, I don't have a lot of experience with pads in the JRB.
Bout what size are you? I am 6'1.5", 210-215 lbs. I have a self inflating Thermarest Camprest ( or something like that) which is 25" wide and 2 or 3 " thick. That is the only wide pad I have been able to try. It def holds the JRB wider apart so that my shoulders do not contact the sides of the hammock, maybe with 1/4" to spare. As you have already experienced, it also makes the hammock less stable, but still workable for me.
I recently tried a 26" wide section ( but only 20" long) of WM blue pad (the non-egg crate style) and my 1st impression was that it did not make much difference in the opening up the hammock. But maybe that was because the pad was only under my shoulders due to the short 20" length? But the wide SI definitely opens things up.
The Exped pads are 1" wider (26") in the long/wide versions. But have not tried them in a BMBH to compare for ability to add width.
What about the new 4 season Neo Air? Can they be had in a wide version?
So, that is all the experience I have with wide pads in a BMBH, not much really. I normally use my JRB MW4 UQ with the JRB Bridge, and no pad. It is not as wide as I would like, but over all I am real comfy with that combo.
So is there no place close by- with good return policies- where you can get a wide SI Thermarest ( or even a wide Exped) and give it a try before you hit the trail?