Jack, thanks in the extreme for doing that: I think we're all trying to work out how to finesse the last drop of performance from this gear and this really helps. I agree with you and Don: the shots are really really good but there is clearly some movement. Not sure if removing the panning plate would help either.
I'm now in the wrong place to look at my test files for the Cube on the Arca L plate and won't be back there for ten days but I just looked up my post and weirdly, despite the Arca L in general not being great when attached vertically, at 1/12th it did the best of the combos I tested. Which leads me to think that there are specifics of shake and resonance in each rig that differ. So probably the only way to work out the ultimate 'best combo' would be to head to head the two plates under identical circumstances. I might order an RRS and give that a go.
I'd say in your shot that there's a one pixel horizontal blur and no vertical - what do you think?
Thanks again Jack!