Awesome!
I assumed that the cap is reduced by 1 for each overloaded part, but its actually the movement cost that is reduced. More precisely, if the move cost is 10 or lower, it is set to 10 and then lowered by 1 for each overloaded part. Now my math checks out.
Except that apparently overloaded hover is capped at 5 speed instead of 10. I guess nobody tried 11+ overloaded hovers (yet).
An interesting effect is that 5 Cld. VTOL Modules with 4 of them overloaded yields 11 speed, while overloading the 5th one more than doubles the speed, reducing the move cost to 5.