[captain harlock]

Well, from my own point of view, you can for example reach let's say 40mph with a 7xl and 12 cells, but you wont be able to reach that speed with a 10xl and 12 cells with the same gearing. Probably you can reach 40mph with a 10xl and 12 cells if you use very tall gearing, but then you'll be tiring up your esc.
So, to reach a 40mph speed run with a 10xl without sacrificing the controller's maxximum performance, you might want to go up in cell counts and look for the most suitable gearing in which both the motor and esc would be running at their highest performance.
IF you specifically want a certain speed, then pick up the motor which would give such a speed at a moderate voltage like a 1940/6 at 17 volts would easily take you to the fourties, but if you want more speed with that motor, then you can get to it with either higher gearing or cell counts, but the controller might suffer from heat and poor effeciency and thus, a higher turn motor with higher cell count can give you higher than 40s speed runs with good effeciency. A 1940/8 with 5s would be much faster than a 1940/6 with 4s.
I might add some more, but I'm afraid I'm not as thorough as a REAL brushless expert. Also, I feel like I'm a bit repeatitive here.