IMHO you would be better off doing it backwards. Use the arduino to read gcode and parse it until you get easy units (motor steps and time steps), then feed these to the fpga and have it do motion control. Fairly easy to get 50MHz resolution on motion control this way, most arduino based printers run at 30~100kHZ.
Fpga are great with parallel realtime stuff, but quite hard to use (and overkill) when you have a slow sequential problem, like text parsing.