Ok just looked at the specs of your motors and the voltage constant is 18.95v/100rpm, I however think that may be a mistake.
Two reasons I am thinking that, first is normally the voltage constant of a servo is expressed as v/1000rpm and second it is giving a figure of 4000rpm at voltage constant, so if the 18.95/100rpm was correct then that would mean you would have to apply 758v to get that.
So we will take it that the specs are wrong and assuming you have a 70v power supply you should get in the region of 3,600rpm for your motors. With a gearing of 3:1 as you have then your rapid speed should be 12,200mm/min. (3600rpm/3 x 10)
However to achieve that rapid you would require a kernel of 2000 (500 x 4 encoder) x 3,600/60 = 120,000Hz or 120KHz
The max possible kernel in Mach is 100KHz but you will need a very fast and stable computer for that, more likely will be 45 to 65KHz.
So to summarise, to get the best out of your machine with that gearing you really need to use an external controller such as the SmoothStepper or CSMIO/IP-M etc as they will allow you to get the pulse rate you require.
Doing away with the gearing may allow you to get reasonable rapids from the parallel port, that will just depend on how much velocity you require and also what torque your machine requires.
Hood