[Anaraxes]

Humans have a "drive train" every bit as independent as the usual centaur's. I don't see a reason to treat them any differently from baseline humans as far as movement goes. Four-legged animals don't have a split maneuver system, nor do birds and other winged creatures.

It's tricky to try to calculate costs based on what maneuvers part of you can't take. What really matters is whether the entire assemblage that is the centaur can take the usual range of maneuvers (or more, or fewer). Imagine a human being built as "only the Legs section can take Move maneuvers" while "Only the torso section can Attack". With the exception of kicking, you've actually lost nothing here. It would be unfair to build each half at -50% for having so few options, and wind up with a huge discount for "Cannot kick". That's on the verge of giving the human a discount for "Ground move only", as the torso+leg sections don't have a wings section, and similar silliness.

If you have a truly separate drive system, with its own intelligence and thoughts about where to go, you can treat it as a vehicle for chosing maneuvers. That works for horse-and-rider, or a tank driver and gunner, or some sort of weird centaur with a hindbrain that moves the body around despite what the head/torso might want to do.