Indeed, summed it up quite nicely as I was going to reply a similar thing to @
erxv
So yeah erxv you're mostly correct, basically it'll come down to the Engine > API > GPU interaction and just how far you can push that. VRAM won't so much be an issue, but just how much of a load the Engine/API can take before it implodes, or so to speak.
That would almost definitely require a proper world/environment editor, like a Redkit, and even then you couldn't be sure as to what positive engine related effect it might have (It'd likely be insane trial and error).