I hear you on the MM3 vs MM4. There is less weight shift on acceleration. But that really depends on the level of grip at your track. We hardly have enough grip at our local clay or dirt tracks to even consider mid motor. Let alone trying to run MM3. They don't sugar or put stuff in the tracks here. Everyone who tries MM here (locally) is too busy trying to keep the back from spinning out so they all revert back to RM. And the times I have ran on carpet using MM is limited, so I am sure Fred is far more knowledge on MM setups.
As for RM setups. RM3 is good on most dirt/clay tracks. But as for the RM4, it does provide more traction than the RM3. When the traction is low, the RM4 will shine more. You can out accelerate your competitors while they gingerly apply the throttle. Not to say that you can't run the RM4 on med grip, it works too but you have to understand that there is a lot of weight transfer happening on the RM4. Under acceleration, the front goes light or wheelies, which is what a lot of people complain about using RM4. You need to move the battery full forward and add anywhere from 30-45g of weight up front to keep the nose down, and you probably want to run 3 anti squat to give you as much off power steering as possible. We also use 10-15 drag brake to assist with steering. Remember our tracks are technical more point and shoot tracks.
Last weekend we had a trophy race and our team placed 3 cars in the A. When they watered the track our advantage was minimal compared to the RM3 cars, perhaps 0.1 or 0.2s per lap, but I think that more of less has to do with our suspension setups and not transmissions. But we were noticeably about 0.5s faster per lap when the track started to dry. When the track was fully blown out, our advantage was closer to a full 1s.
The key is to find a setup that is balanced and keeps the car consistent whether its wet or dry. What I mean about consistent, is that the car won't push or over-steer regardless the condition of the track. If it pushes in wet, and over-steers in the dry, then the setup isn't balanced. You can get any setup to work RM3 or RM4, you just have to understand the benefits and drawback of each and work around it to find that happy harmony of setup that the car likes.
If you have a track the is consistent at med grip, then perhaps RM3 is better as there is less work to do to get that setup. But on tracks where the conditions change round to round, and especially outdoor tracks - having a car that can provide max grip is better. So when they water I don't have an advantage and we are at par, but as the track dries, I slow get quicker and am able to push harder and charge and force others to drive aggressively and hope they spin out on exit.