The 7xl will require a 18t pinion to get over 45. The 6xl will need a 15t pinion to do the same. My issue is that on 4s the 7xl is spinning around 35k rpm which is good. The 6xl is spinning over 41k rpms which will cause more heat. I feel that the 7xl with the 18t pinion will run cooler and give better runtimes than the 6xl will. Just my opinion though. I had a mbx buggy, which is the same thing as the gtp drivetrain-wise. I ran a 7xl and MM on 4s and it ran great for me. Make sure that you use the rcm heatsink/clamp on the feigao motors. It helps keep them cooler and stops the endbells from coming off.