Anyone?
simul against 2300

I don't know. I played against a strong FM who played about 30 people at once. We all lost. At the time I was 1750ish USCF and probably the highest rated player.

I would guess that playing that many games at once might drop a player's performance 100-200 at most, if you are lucky. So say, 2100 to have an equal winning chance. But that is just a wild guess.

In my experience it really depends on who the 2300 player is. Some masters are better at simuls than others. It also depends on who the other players are. If most of them are beginners the master will be fresh and crisp when he gets to your board; if many are strong players he will be tired and distracted. If you get to choose your board, sit immediately "downstream" of the strongest player you can!
I had a draw versus 2200 when I was about 1700, in a 20 board simul, but I'd have put my chances well under 50/50. (The other players in that one were weaker.) I'd say that 2000 would give you an excellent chance of beating 2200 in a simul, so maybe (no personal experience of this) 2100 to beat 2300+. If you stack the boards with strong players, you might get wins at 2000.
Now I'm evilly tempted to round up the strong kids from my local club and have them pretend to be beginners against someone who doesn't know them. We have a couple of 2200+ 14-year-olds and 1900+ 9-year-olds who would give someone a very bad day. Probably have to leave the state, though: one of the 14-year-olds is State Champion, and anyone likely to play here will know him!
(I have an evil streak which manifests in watching skittles games in parks or malls and saying, when asked if I play, "Uh, sure, could I try?" and then beating up unsuspecting casual players. I know it's immature but I never get tired of it. People just do not think that the gray-haired lady might be a strong player. To twist the knife I like to play Bird's Opening....)
what rating do you need to get a equal winning change against a +2300 rated player who is playing against 20 to 30 poeple at the same time wiht white?