What makes you so sure that current gen chips are underclockable to better than advertised efficiency? Like how bitmine advertised low power mode at 0.35w/gh (according to your understanding of physics it should have worked) yet underclocking did diddly squat.
Underclocking alone indeed does "diddly squat" to power efficiency. The key is lowering the core voltage. If core voltage can not be raised/lowered to achieve higher speed/better efficiency, that generally hints at a design flaw. I have no idea whats going on with bitmine specifically. Got a link showing that power effiency does not increase quadratic with voltage?
You need some evidence before you can claim so confidently that all current gen chips are underclockable to below 0.4w/gh (at a reasonable $/gh)
I dont, because I never made that claim.
How exactly is spending 10 million rushing to the smallest node size cheaper?
If you dont understand the difference between NRE and per GH production cost, there is not much I can do. Not that $10M sounds realistic to me, for a chip as simple as a bitcoin miner. Its not going to have 15 metal layers like a highend CPU or GPU. Id be surprised if it has more than 3, maybe 4.