Personally, I think that a 7XL on 6S is too much. I also think that the 6XL on 4S is too much. The 7XL on 4S would be about perfect for a MT or buggy, if you want speed. Otherwise, the 8XL on 4S is very practical on 4S giving good performance and speed. What are you going to run this in? If you are really going to be pushing the motors like that, you might want to consider a "better" motor, like a Lehner or Neu.