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.
So, I raised my voltages somewhat (920-930) and used cast-xmr. I did a straight 24h run with no issues. 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 then reboot the rig and with the same settings I started stak-amd: within 30 minutes one card stopped mining and the whole rig hung. There is something definitely bad that stak amd brings up from my rig, but still cannot pinpoint it.
Now I'm back to lower voltages on cast xmr to see if it's stable.