are you on autofan? i see HiveOs you have is also old verison. is this version that you have the best one?
Your cards appear not bios modded right? for instance. the ones with Samsung memory should do 31-32mhs.
i would mod them with polaris bios editor and ubermix 3.2 version of the timings and then i would use settings like core clock 1150, core voltage 850 , memory controller voltage 800, memory clock 2100 , ref rate 30 on all . this should get you 31-32mhs.
Another thing with cards is the sillicon lottary which can play a part in heat, watts usage.
if you want to try reduce heat and watts used you can try lowering the core voltage on one card by values of 5 to see when it gets dead and then increase just a bit to make it run again.
for instance. i have RX 570 with Samsung memory its from Asrock.
it does 32.22 mhs but with 104 watts coz sillicon sucks. my settings are 1168 core clock 862 core voltage 800 memory controller voltage 2100 memory clock. ref rate 30 , aggressive undervolting switched ON.