Erik for these modern tanks the three to ones seem to work well. The stock gearbox was 30 to 1 and your are 39 to 1 so you do get a 30% lower gear to pull the extra weight and the better motors to keep most the speed. The lipo does help a good bit also. I also have the Tiagen electronics and I think that also contributes to the drivability as it seems to control the motors better than the henlong. Next will be a better control board and the 9X. It looks like the new henglong stuff does a better job than the old controllers. Hopefully the OP will keep us informed as the upgrades progress.
Jimmy