Going from a 24 volt power supply to a 48 volt supply should permit you to increase the motor speed (and have better torque at higher speeds). However there are other factors that can affect the maximum obtainable speed from your motors (such as resonance effects). According to the Keling spec sheet, the motor winding inductance (when wired in the bipolar parallel hookup) is 6.8 mH, which is a relatively high inductance. Using Gecko's formula for choosing the driver voltage to optimize motor speed, it would appear that you would like to use an even higher voltage than 48 volts (perhaps in the neighborhood of 80 volts, which is greater than the G540 is rated to handle). But 24 volts is somewhat low if improved speed is your goal.
You will probably not see a big difference in speed if you merely replace the Xylotex with the G540 and continue to use the 24 volt supply, but you should see a speed boost with a G540 run at 48 volts, and a potentially bigger speed boost with a different Gecko drive at a higher voltage.
It may not be a good idea to drive the G540 from the C11. The C11 has optoisolators between the PC's parallel port and its output pins, and the G540 also has an optoisolated interface to the PC. Using two optoisolators inline is probably not good practice - for example, it could slow down or otherwise distort the step pulses, which could possibly result in unreliable operation such as missed steps or other problems.