I know this probably doesn't help you but I'm running the new driver 397.31 and both of my cards (nearly Identical to yours) are running 96%+ when rendering.
Dual 1080 TI's with Win 10 and Visualize 2018 SP2
One thing you may try is to use Display Driver Uninstaller (DDU is Free, Google "DDU") to completely uninstall all display drivers and any driver remnants. Then do a clean install of 397.31. I've had very good luck using DDU when having display driver issues.