I failed to measure current at 40GH - I can check that next time I try it.

35GH/s was 39A@0.975V = 38W.

I did try a longer run under bfgminer and was seeing some hardware errors (about 0.5%). Not sure why my test wasn't catching them - I was just running zefir's test vector over and over (and validating that the correct nonces were returned). Guess I need more test vectors

The board did start getting pretty hot. I have a water block attached to the bottom of the board but just a heatsink on top of the chips. The heatsink was sitting at around 35C but the board (the top) itself was getting to over 60C. I suspect I need more via's to better transfer overall heat to the bottom of the board. Or figure out a heatsink that can cover the board top itself.

Clearly immersion is the way to go