This AMD site shows how to setup WATTMAN: OC+ for many cards... including my RX580 OC+
This provided me the best performance, but it's now obvious my UltraWide LG 2560x1080 monitor is too much for this card to handle with free-sync... or it could be the monitor is too old and needs a beefy card like VEGA56 or VEGA64.
If I want to use this card, i cannot use the DisplayPort... even if I disable Free-Sync and GPU scaling, the screen still flashes on/off and a 1 inch horizontal bar appears occasionally. Just switching to HDMI solves the problem.
I still like the card but I'll switch it to another PC that has another monitor.