One of my friends was talking with Javier Castro (the one who built the R4 cars here), and he said that the difference between the R4, R5 and MR is that the MR have a longer suspension travel that complicate the setup, and that the ARB could help to give the dampers/springs to have more freedom to be setup.
About diffs, I still have to get more information to know what kind of LSD the MR uses, and if there is some freedom to improve them.