I have no problem with MPH, other than it needs coding
When I get there, I will have it configurable - a custom name, a custom multiplier (from m/s, of course). Just as anybody can make a car that shows speed as number of fallen trees per time it takes Scotsman to drink a pint of ale, anyone should be able to use their desired units on the HUD as well. But internally, API wise, there will be just one type of units used by the physics, because it would be hell otherwise.
Actually, the whole HUD thing is going to be configurable in the future, since there are custom HUDs in many aircraft that need a universal approach.