My mini mill has a belt drive which has had me contemplating a conversion for the 727. I haven't found the 6 speeds to be too limiting, but I do wish I had a speed around 450. The gap between 320 and 600 would be nicely filled with that one more speed IMHO. The one issue that would have to be addressed would be the quill travel. When the quill is lowered, is is fully engulfed down into the housing. You would have to figure out a way to drive the quill. At the same time, you would still need to be able to do tool changes, ideally without having to raise the quill each time.