Ok, so if "system" is only the "rules", as in, things you CAN and CANNOT do in a game, and HOW you do and resolve those things that you can do, in other words, game mechanics, then we, at least, are agreeing entirely.
We may put that to bed, absolutely. No matter what we find, we are in agreement that at the very least "System", as we'll be using it, is that definition. Spot on, hoss, spot on.
Now I'll repeat my earlier statement: You can't build an identical gameworld with different systems, because the gameworld must match the mechanics. (That being said there are a ton of games being sold where mechanics and world are not sepparated well enough and the game-world doesn't match the mechanics at all.)
"Gameworld". That's a tough one. What are examples of "gameworld"? I imagine metaphysics are (games where there are supernatural elements are different than games without them, due to supernatural element rules). Definitely physics are (the amount of "damage" one takes from a "fall" or a bullet, or how much things weigh in D&D). But are "pointy ears on elves" or "accents" or names for things a part of the gameworld?
A stark example would be--and here I wear my VtM roots on my sleeve, I know--that I don't think we're disagreeing that a change in the mechanics of vampirism would change the gameworld for Vampires in V:tM. There might be a threshold for that, though, small changes yielding small divergeances in the gameworld, big ones making it almost unrecognizable--but not matter how little or much, its different... and that's the point.
I lean heavily towards the idea that if you change the system, you're likely going to change the world--yes.
Now experience, which is a concoction of play style, genre, scope, and a few other aspects is based largely on gameworld. I'll hand you that the second biggest contributor to experience, if not even surpassing the first, is entirely irrelevant of the system, and that is your fellow PLAYERS. But that's not something we're talking about here.
Well, yes and no, boss.
Experience might rely on the gameworld for many things (ranging from flow to genre to style to "how" and whatnot), but I think its safe to say it also relies (in part) on Story (narration, perhaps?), Participants (the human factor?), and Environment (the RL when/where/many-other-variables). To what degree someone's experience relied on each of the four (Gameworld, Story, Participants, and Environment), I just don't know.
Someone who is far more interested in Story might not notice a bit any divergeance in the Gameworld or much care who played. Someone being a literalist on Game might not make note or walk away after with any real involvement in Environment. If, like some of my more colorful ex-ladygirls, one couldn't care less about what was going on or how it was going on or under what circumstances it were going on might just be there for the people.
I suspect in the same game, those three might relate very, very different experiences. As such, its not meaningless, is it, to take into account not just "what occured in all comprehensive detail" but also "what everyone experienced"?
In which case, for a given group of people, might there not be a different experience with the same System (the torchbearer for Gameworld)? And if that's possible, a similar experience with different ones?