Was wondering that myself, it is 2:1 at the moment, not sure what I have the driven pulley bored out to but I may be able to swap around so it was 1:2 which would give me 5mm per motor rev rather than the 1.25mm it is at the moment. Whether the motor would be good enough though is another matter as it is a tiny servo, 50w if I recall.
I have also found a lead screw with 8mm lead but not sure how the friction compared to a ballscrew would play with the wee servo. Probably worth a try though as it is dirt cheap and shouldn't need much in the way of alterations.