I did not watch the red LED frequency with mine stacks, but what prevented them from reaching 400GHps turned out to be insufficient power. With 850W PSUs I was not able to reach more than 360 (and also had USB errors, btw.), now with a 1200W PSU everything works smooth and each board is at 50GHps on average (1080mV, 275MHz). At wall I measure around 750W draw, but there might be spikes that a PSU operated at its edge can't handle.

What you can try to double check is to operate only 4 boards at a time and check they reach 200GHps (terminate CAN bus correctly + unplug ribbon cable from inactive ones). If so, you need a stronger PSU.