Circus Galop: https://www.youtube.com/watch?v=DekTSh1QmvY
Death Waltz: https://www.youtube.com/watch?v=tds0qoxWVss
Both get a little carried away, but the main melody is clear and pretty interesting.
I imagine our hard limit is human attention. We can focus on the lead and to a lesser extent listen to the background stuff but probably not listen to much more. I don't think we can really focus on some large complexity. That just ends up sounding like a discordant cacophony to us. If anything, we're made to enjoy fairly simple musical relationships. Chords can only get so complex before they sound off to us.
It seems to be a happy little coincidence that ten fingers is enough to make complex music without hitting too many practical limits. Just because that limit exist doesn't mean its wrong and that we need to overcome it. The same way a third leg won't let us run faster, fight better, or climb better.
https://www.youtube.com/watch?v=qMHNrRaQB4Q
Audio's not good, alas, but it's actually being played by a human. It sounds like it's got all the complexity of the machine-played one, which of course it doesn't --- which does suggest that the extra complexity wasn't adding anything in the first place...
In case it wasn't clear, "Death Waltz" is just a cover of "U.N. Owen Was Her?" from the Touhou games (as are many of the more popular black MIDI songs from what I've seen).
https://www.youtube.com/watch?v=SyC5eWJhCr8&list=PL_I1U99bY0...
Circus Galop was cool, though. That's the first original black MIDI piece I've enjoyed so far.