I've also heard that GEOS does (in fact I seem to remember a magazine article at the time touting that as one of the reasons GEOS was so fast), but I've also been told it will work with the SuperCPU. I don't think both can be true.

I wonder how well the emulators emulate the undocumented opcodes. That would be a good way to find out.

Successful C64 emulators certainly emulate all the illegals and many of the hardware quirks, or else too much software wouldn't run on them.

GEOS has been reverse engineered into a code base that assembles byte-for-byte with the originals, so you could scan it for illegals: https://github.com/mist64/geos

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WFDis Interactive 6502 Disassembler
AcheronVM: A Reconfigurable 16-bit Virtual CPU for the 6502 Microprocessor

That's true. As an avid collector of vintage hardware, I lean on the side of traditionalism. Meaning, keep the original hardware as close to the original as possible. However, I bend for certain conveniences such as modern storage, better displays, etc.

Some people are so uptight about it, they won't even take the yellow off the plastic. Because, it "was meant to be". lol

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Cat; the other white meat.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

I made one which seems to be faster than the SuperCPU but I'm still waiting on the rich and famous part...

https://microcorelabs.wordpress.com/2021/04/19/mcl64-worlds-fastest-commodore-64/