Damn, im jealous! I want one of those very badly! I looked on abay a while back for a roller but just didnt see any. I wouild throw in your MMM and see how that works before putting the MMMXL in.
The bad rear bulkheads are almost exclusively on the gasser. It doesn't have the rear bracing like the XB-E does. The ones on the gasser were melting / breaking around the rear diff pinion area. I've not heard of anyone having the XB-E rear bulkheads break.
I have an XB-E also. I will be running it with a 5692 730KV Leopard on 12S with a FighterCat 16S 300A ESC. The motor is in the mail.
Be sure to shim both diffs I had quite a bit of play in mine.