Actually, the plasma is pretty fast...up to a point. I'm not sure about the thicknesses you mentioned, but we had a demo unit to try out that used a special tip on the plasma torch for gouging. It was very easy to use, the gouge was clean, and travel speeds were pretty fast. (Even I was able to make clean gouges with it.) You could wash it side to side for wide gouges or dig in for deeper grooves. We didn't try it on anything over 5/8 T though.
It is more expensive, and more prone to breakage that air arc.