why don't you just use regular roux. No set ups and recognition is quicker. I can sub-3.5 step-4. It is possible to sub-3.5 using M2 for the last step? considering the amount of set ups, no you cannot. The point of roux is reduce the cube to just using <M, U>.
I agree, step 4 takes my less than 15 moves on average. When speedsolving, even with a fractured finger, I can sub 5 step 4. M2 would be horrible for Roux.