I have lots of notes marking people as being on the leaderboard for X month, etc... and if I play against them a lot, specific notes on their play...

But at the higher buy-in SNGs, I usually feel fine as long as the table doesn't have more than 5 other people that I have noted as good/decent.

I don't think that 1 player, no matter how good they are, really decreases your earn much...