Interesting topic. When I ran my MTX4 last weekend it was hands down far more punchy than I ever was able to get my R40. I am not really sure why that is yet, I am still doing some investigation of final drive ratios and the like. The MTX4 uses a different internal gear ratio, and the clutch bell is a little smaller and lighter than the MTX3. I will post my findings on the FDR when I get them, I suspect it is mainly a gear ratio thing.
For the R40, the MTX3 clutch is the way to go for sure.