"It also depends on where you are in the ladder. Getting an 80% win rate isn't too hard when you're in the bottom third of the ladder but it's presumably tougher when you face people in the top third, since in both cases you need to be underrated to have that high of a win rate, and you're much more likely to be underrated if you're near the bottom of the ladder. In any case, someone who wins 80% of their games in the top 100 would probably easily crush someone who wins 80% of their games in the bottom 100."

There's a bit of faulty logic here. I get what you're saying, but if you win 80% you won't be in the bottom third unless you rigged it by losing a ton intentionally to cripple your elo rating. Winning raises rating, as raiting raises you play better opponents, thus anyone that wins 80% won't be in the bottom third. Everyone should win roughly 50% of their ladder games. Only exception are people at the top who can beat almost anyone over 50% of the time.