Not very familliar, (yet) with MR, but is there some sort of possible "user-controlled-photon-cap" setting in the MR settings that needs to be upped in your case maybe?
I only guess, so don't take my word for it, but I had the idea from seeing the Subd poly conversion cap that was user-controlled.
...I'm most likely full of it, but it's just a thought
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS