Is the default speed 0 or 50?
Documentation seems wrong on both the lexicon and BioWare.
1) The variable you are setting is bounded by a minimum of -100 and a maximum of +50.
2) The variable represents the speed increase as a percentage (e.g. setting a value of +25 will cause the horse to move 25% faster than the base creature speed (mounted appearances move at the "fast" speed). A value of -25 will cause the mount to move at 75% the base creature speed (75% is 25 short of 100%).
3) If the variable is not set the game will assign a value of +50 for the speed increase, so an unset mount will get the maximum speed benefit.
4) Monk and barbarian levels can reduce the speed bonus, but with the bug of speed stacking this will still allow them to move faster than other classes without the speed feat.
5) There is a hardcoded cap on movement speed effects at 50% faster than the base speed.