Hi Alex, please refer to the operating characteristics table on this page for the actuator speeds: http://www.gimsonrobotics.co.uk/GLA-S_l ... uator.html
You'll see that at the nominal voltage the 14.4V model will travel at 34mm/s when unloaded.

I haven't been able to find 18V or 24V nominal versions of the 550 motor used widely available, despite having looked all over the place- the few suppliers who might have a viable motor require minimum orders in the hundreds so it's not really an option for this market. You should find that with the right conditions (heat-sinking/fan cooling and/or current limiting) you could run the 14.4V motor at as high as 21V, but you'd have to be careful- certainly not do what I've seen some people do and tape up the cooling vents on the motors!
That's if you're after extra power and speed though, if you've just got a high voltage robot and want to use GR01's then just set the travel limits on the transmitter to 3/4'ers or so on the driving channels, so that the motors will never receive the full supply voltage from the speed controllers.