My only issue with 13S is the odd pgroup and weird stacking required. However, if you have room for 40 cells, why not just go with 39? Bamboo sleeper anyone?
13S is fine. The FocBoxes can handle it fine. 13S will be pushing it for the Unity, but the Unity will also handle 12S perfectly well on delivery day. 13S will not run well on any MayTech 4.12 derivative to my knowledge. The VESC6 derivatives may be able to handle it. We’ll probably burn the FlipSkys up at 13S in the coming months.
I’ve also heard rumors that on the focboxes the ERPM limit is a myth. In fact, drunken discussions were had involving an experiment with focboxes and a set of NTM 270s at 12S. There may be a reality check on the soon.
My personal belief is that 12S is more efficient and runs cooler. That’s my observation, Math may say something different.