The downside of a light gantry is you get flex over the length. If you drive a 6 ft long lever from one end and do the acceleration you need in plasma then you can see inertial forces at work with some ugly cuts. The solution is to drive both ends of the ganty. You do that either mechanically with a cross shaft and one big motor or twin motors "slaved".
The MACH question has to do with the inevitable question about what you do about Torch Height on that large a cut surface. It's disappointing to find you saved money on the build only to find you can't take advantage of the cost effective THC options.