They don't use more power for the distance you travel. 50% more speed for 50% extra power per second, so you're still spending 1.5 power for 1.5 progress. Whereas the mining motors cost 2.5 power for 2.0 progress (25% more power).
I have tweaked some of the wording to make this more apparent, but it may need some more work once more of the bugs are out of the way.