The lowest-level contour I could consistently trace is for RMSD value of 1.8, and that holds for almost the entire length. But the dominant finding is how level it is. It would be like finding a gorge you could walk along the bottom of for tens of miles without any change of height. This landscape has implications, both practical and theoretical, for 2D parameter search. In this case, they don't.
Comments are welcome.
I'll try to write up a report on this investigation and make it available, thought it's hard to see how it could be published since it would be of interest to hardly anyone outside the ACT community.