Further observations over the last 12 hours or so lead me to suspect it's a combination of increased structural stability minimizing card-wobble, and lower ambient temperature that is allowing the gains. Was fairly solid overnight near 570GHs in cooler temperatures, and down to ~562-565GHs during the day now with ambient a few degrees warmer. I'd love to write it off as a placebo-effect, but I've tripped an OCD trigger, and it's game on now trying to unravel this performance curiosity. More tinkering over the weekend...