Are you sure those QuickFists will handle the shifting weight of rack and gear over a rough trail? I wouldn't be comfortable with rubber clamps holding a roof rack full of gear, even a half rack like that one.
Excellent question. I'm planning for a max of about 300 lbs with a working limit of about 200 lbs.
The Go-between model is good for 50 lbs with a break strength of 150 lbs. They are rated for holding tools or whatever vertically. Most of the weight here is not vertical, but simply sitting on top of the clamp. They are very high durometer rubber. The exception would be climbing a rocky incline where the rack would be trying to pull backwards. In this case the roll bar pad will keep it from sliding backwards as well as the friction of the rubber on the roll bar. I'm guessing 300 lbs in that direction would not be a problem as it's still mostly pushing against the bar with maybe 50 lbs on each side trying to shear off the roll bar pad. This is well within my estimated limits.
The rack itself is rated for 500 lbs in the original configuration, attached to a receiver. My PVC spacers may be the weak link, but even with 300 lbs in the rack, each PVC end is limited to about 30 lbs of torque (guess), so I have no current plans to use steel tubing there.
With 300 lbs in the rack, and the rig on it's side, the rubber mounts will have to hold 75 lbs each. Over limit, but well within break strength.
The rack is very solid. I'm not going to stress test it, but I'm sure I could stand on it, and I really have no plans to load it up to 300 lbs. I will put all my soft stuff up there (chairs, tent, maybe the stove) and keep my ice chest and water can down in the bed. Worst case will be a Dusy Ershim trip where I'll have to put two gas cans up top. That's 60 lbs and only if I bring the dog. If it's just me and a passenger the gas cans go in the back too.
I still have plans for a tire rack and there may be room for a can rack there too. Not sure about that yet.
Over the weekend I finished up the hardware. Each clamp is bolted to a flat plate with two screws where one of the screws passes through the rack, so the weight is distributed across the two screws. I also added a switch panel in the corner behind the driver and a dome-light panel in the center with an OEM dome light. No pics though...