I'll look into it but since the numbers work out for us for now (fortunately), I'll keep it at a lower priority. Having a good science reason will be good, I agree.
I couldn't think of a strong argument against having extra channels. Plus we don't really know what temperatures the cavities go to (and will go to with new heat shield on). So for the first version at least, I wanted to keep extra channels and decide this answer in the future.
Unfortunately, I think the present version won't really work without a lot of ugly scratches on the pcb. I messed up this one. Thanks to JLCPCB, getting boards is cheaper and faster now. So I'll get to this right away to avoid Christmas delays.