G0 is a full rate move so you would have to set the Mach 3 to limit the maximum IPM of your machine as that is the value used for the full rate movement. If you want a slower rate of movement back to the zero location you'd have to use a feed rate or G1 move to accomplish this. The problem you'll have is that the Safe Z movement will put the control in a G0 state so the you would have to issue something like G1 X0y0 F40 for example to put the control back in to a G1 state and the F40 would be the feed rate.