I guess Walt_F4 has never tried cav
With riding affecting both top speed and acceleration, you could do some different tests like time between fixed points when at top speed, and then time from a stopped point to a fixed point. I suppose maneuverability could be some kind of turning radius test when at full speed, hard to measure though. Almost need people of different skills on the same server with the same horses, have everybody do a full speed turn and see where they end up.
You could do a fixed course with a series of turns for overall time (like air racing), but the variability would seem to be high without a huge number of runs, can't see the small % difference very easily.