First take your current setup with any belt reduction and leadscrew pitch and calculate the speed you can get now. Once you have that number in IPM of travel then multiply it by 2 (using 70 VDC as a rating for the Gecko) That will then tell you the max you can expect. You won't lose torque (which comes from current) but you will have basically half the upper speed. Is it enough? depends on if you are using more than 1/2 the speed currently....
Another approach would be to replace the motors with ones that have higher RPM rating at the 70 VDC target. The 850 oz-in we use in our Complete Packages are rated at over 3000 RPM at 72 volts DC. Put in the same place as the older motors and using a new power supply (which you HAVE to do with the Gecko's) you will actually get more speed.
In the end, dollar wise it might be better to just go with the higher voltage drives. and everything remains the same (except the control software)
Totally Modular CNC Electronics