We tested rear motor on the track used for the euros over here in the UK. That day the track was fresh and slippy and both mm and rm turned a similar lap time, but the mm was easier to drive. MM does one thing say oversteer a little, but the rm can oversteer then push through the same corner.
When the Euros ran we used extra calcium and a bit of sugar to bring the grip up so it was different to the warm up race, so everyone ran mm.
Maybe an extra class is needed, I do like the look of rm cars.