I ran into something else interesting along the way the other day when I got the last pair of these. I had been running them on a USB2 bus and the 6 were always fine at full speed. After stepping up to 8 of them, I'd consistently see one slow down after a while. Moving them over to the USB3 bus of the same system fixed that. I wouldn't think the USB2 bus was saturated, so I don't really know why.
* Are you using external fans?? I could not see it so quickly from your picture. If not, have you measured your ASIC temperatur (small heatsink)?
* Why are you using 4 of these hubs with just 2 MLDs each if you had both a USB2 and a USB3 hub that (I understand from your message) could take all 8 in one go?? (where only the USB3 gives all units at full speed)?
* I have similar issues with running a certain number of MLDs just fine, and adding one or two more one or two units slowing down. It has been suggested it is an issue with USB hub getting 'stuck'. However, I am close to certain that on my hub it is a power issue and it could be that the powerlines in your USB 3 hub are either fatter or differently laid out on the PCB. It maybe takes a small additional voltage drop to cause problems. These babies draw a lot of current. That's my experience. But whatever the cause, I have seen similar things too.
Great stuff. I am running 6 at 924Mhz and another 5 at 832Mhz (for the above reasons; all on one USB3 hub). Getting about the same as you, slightly less. I also have slightly higher hw rates (but then I tuned core voltage down as much as possible to reduce power and heat).