I think they still should go much faster then I can get them to go
YES, but your voltage limited, the time required to get the current level up ( think of an RC circuit, becomes inductive rather than resistive ) is dependant on the voltage which affects possible motor speed. They don't give your curves for different voltage levels other than say 24 volts. As the pulsing is increased, your pulsing / waveform being sent to the motor reaches a point where it is very unsatisfactory and thus you reach a max motor rpm for a system. SYSTEM means your power supply, drive,and mechanical / machine, thus, all have an effect on the preformace.
You may want to have a look on the Gecko site as they have a lot of theory / info explained better than i could ever present .
RICH