If you're into 6502 and more specifically the Commodore 64, you should give the winning demo from last weekends Fjälldata a look. Quite insane stuff they do nowadays... https://www.youtube.com/watch?v=WtpPPgFSwxE
I feel this was an example of the crazy things you can do with PAL C64 hardware and not necessarily what can be done with a 6502. I feel Apple // demos are more representative of what a 6502 can do (without hardware assisted sprites, character generators, ADSR waveform generators, etc.)
It's kinda amazing that the people who wrote this probably know every single byte of code that's running on that computer, rom included. Without such deep knowledge you can't get this kind of performance.
With a modern computer that is simply unthinkable.
C64 demo coders don't generally care about ROM contents, there's nothing useful in there. ROMs are usually switched off. Although there might be a few exceptions.