Wrt GFX: Most demos, certainly when thinking about the 4K challenges, would be using nothing more than some variant of a basic VGA display mode (usually but not always Mode 13h or some variant like “Mode X”) without detecting and taking advantage of acceleration features - the difference between graphics cards at this point was not nearly as significant as what the demos were doing in the CPU, even the nastiest ISA card of the era is going to keep up at a reasonable framerate, IIRC from my doom playing days.
At this point you can be talking about 600 kB on a DOS machine. Sky is the limit on modern systems.
That said, typical inner loops of that era demos were pretty small, usually 20-200 byte range, IIRC.
Pre-cache era demos often had huge unrolled self-modifying speedcode inner loops, but that's another matter.
Speaking as a 4k demo coder in the 2000s era, I think you're over generalizing. Decompression sure, but after that all bets are off.
What matters is how it performs on the party machine
Even with a cycle accurate CPU you'd still have a lot of variation caused by the motherboard chipsets, DRAM speeds, cache chips and graphics cards.
Tseng ET4000 was on a completely different level than something like Cirrus Logic CL-GD510 or heaven forbid, Oak Technologies card.
If the era appropriate PCs had huge variations, what extra does cycle accuracy really bring at this point? The same PC demo could perform very differently on two 486DX2 66 MHz PCs.