800 point difference of course he can win in a certain time frame, even if they refuse to resign, because he will only need a fraction of a second to keep making moves. With stronger players, they will resign promptly, but he will have to spend more time per move.
I've never played a simul. How does it work when you're the last board? Can you take as much time as you want to move?
Kasparov is a businessman and I suspect that anything is negotiable for the right price. Want a simul with over 2000 rating allowed? The price goes up.
I read his general restriction on simuls and in the info on his/his agent's website is boilerplate. It's an upfront disclosure to the masses of media relations and PR folks who are looking for info to book him at events.
What do you think is more important to Kasparov---a few more dollars for simuls or the fact he could tell his grandchildren about his fabulous won and lost record in simuls (which he acquired by limiting players to elo 1999 or below.)
There was one reason and one reason only for this rule---improving his stats!