With the supplied tripod, about 8 pounds, max. With a more robust tripod, maybe 12 - 13 pounds. I'd set the upper limit at a C6 or similar.
Is it really just the tripod that is the limiting factor? I was thinking of the cube pro for my 6" Mak but the ota alone is 12lbs add on a finder, an eyepiece or camera and 13-14lbs wouldn't be surprising so I had ruled out anything less than the mini-tower (which is out of my current budget).
I could sell the Mak and get a C6 which are quite inexpensive right now and a couple of lbs lighter, but all the advice I get is that it won't give as good an image as the Mak so I'd rather not.
I would have thought it was the bearings, gears etc. that limited the maximum load (at least as much as the tripod)... or is the head beefier than its rated capacity would suggest? If so that would be great!