I don't really have a clear opinion about whether low-rated players should play in opens or not, depends on so many factors. But as for what Fargo rate can realistically beat a Fargo 800 player in a race-to-9, 9-ball match, I'd say something like:
500 = 1/100,000 (Practically never going to happen.)
550 = 1/5,000 (Extremely rare.)
600 = 1/500 (Very rare, but starts feeling realistic if the 600 plays his A+ game, the 800 is off his game and the rolls+breaks favor A heavily.)
650 = 1/100 (Rare but possible, 650 are already very capable players and on a given day they might play closer to 700 speed. Still requires heavy rolls from the 650 and weak play from the 800.)
700 = 1/20 (Still a big upset but no longer miraculous at all. These happen from time to time, as 700s are very capable players. This is the first case where extreme luck isn't required from the lower rated player to win a race-to-9, all it takes is an on-day from the 700 to play like a 740 and an off day from the 800 to play like a 760 and its almost even.)
750 = 1/5 (No longer a big upset, even if the odds favor the 800 these sort of results happen all the time. It's comparable to e.g. Joshua Filler losing to a 810 Fargo (860 vs. 810).
Given these subjective estimations, i'd say 700 is the first one that has a real fighting chance without the win feeling like a miracle. 650 can still win on heavy luck + overperforming their rating but these will happen maybe 0-1 times on average in a large tournament, so the odds of a 650 going deep in the tournament is still practically zero.
As for what rating starts having realistic (rare, but possible) odds of making it to say top 16 or top 8 in a WNT major, maybe somewhere inbetween 750-780. And for the rating for actually winning one, definitely 780+ at least.
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