Most recent GPU I've fully unlocked to a higher model was a Radeon HD R9 290, which took a 290X bios and became a full fledged 290. There have been a few cases like this with more recent GPUs, as board partners still occasionally flash higher-end boards to lower-end cards to make up for supply deficiencies in the lower-end models. It's certainly uncommon though.
Anyway, I fully expect the disabled portions of the GPU to be fused off and impossible to reenable with the RTX 3000 series, and I expect different physical memory ICs on the 3080 and 3090 boards as well as two unpopulated memory channels on the 3080.
If the memory tables are compatable, flashing 3090 firmware onto a 3080 could still be the easiest way to increase the maximum power limit of a 3080, without having to resort to hardware mods.
The board power requirements are not all that different from prior generations, and should be just as tunable. Idle power consumption is also negligible and performance per watt has certainly increased. You don't need an 850w PSU for a high-end system with a 3090, and even if you did, it's not going to draw anywhere near peak loads the overwhelming majority of the time.
We don't know how different these processes actually are in efficiency as no GPUs made on Samsung's 7nm process have yet been reviewed.
Not practical configurations. The GPU has a 384-bit memory interface which mandates 12/24GiB in fully enabled configurations. ROP clusters are almost certainly still tied to the memory controllers as well, making it impractical to cut them down without disabling memory channels.
The 3090 has 24GiB to make sure it has a bigger number on it than the competitor's part. People will buy 24GiB cards at the high-end even though it's a meaningless figure and by the time any game benefits appreciably from more than twelve, the 3090 will be quite long in the tooth.
The 3080 has 10GiB because it's only got ten of twelve memory channels enabled and putting 20GiB on the Founders Edition would be an extra cost on the BoM and would eat into AIB's options for making distinctive non-reference products.