Thanks for mentioning z386. Much of its recent progress was made possible by the reverse engineering and disassembly of the original 80386 microcode (
https://news.ycombinator.com/item?id=48247004), which z386 uses as its control program. That work has been especially valuable for reproducing obscure protected-mode behavior correctly.
With correctness helped greatly by that, development is now focused mainly on performance. In Doom, z386 is currently about 30% faster than ao486 on the same MiSTer setup, with benchmark performance roughly comparable to a 486DX2-66.