Thanks for the data collection - watched it live!  I received my Vega 64 today (powercolor reference) and tried many different combinations.  What I found worked best was to just to use wattman (not many choices) and play around with the combinations like you did.  I ended up tuning until the activity bar at the top of wattman was nice and smooth with no bouncing around (I am so technical).  For me this was 1401 MHz GPU - Memory 945MHz - Fan speed set to maximum (4561 RPM), temp was hovering around 61C and power limit -25%.  This gave me stable 37Mh/s (-dcr 10) at 276W at the wall.  Will see if it's still running in the morning only 25 min. into just letting it hash.