I also use Sapphire cards and yes stak-xmr uses more power than cast-xmr, I noticed also 
If you lower the speed of HBM it's useless.
I found why my rig was shutting down (I think), I changed the power plan, monitor -> turn off - never; HDD -> turn off - never, and so on... Without these setting many times after 30 - 60 min since I closed the connection thru teamviewer, my rig either was shutting down or froze. I see now it's working, hope this was the issue.
I've raised the voltages to 920mV and 930mV on the problematic card, both on the GPU and HBM. I had the always on settings already, but had overseen the linkstate power management on the pci-e bus. If you lower the speed of HBM it's useless. I found why my rig was shutting down (I think), I changed the power plan, monitor -> turn off - never; HDD -> turn off - never, and so on... Without these setting many times after 30 - 60 min since I closed the connection thru teamviewer, my rig either was shutting down or froze. I see now it's working, hope this was the issue.
Let's see how it goes. I'm up 30W from the wall (running at 720W for the whole rig of 4 vega56).