I've recently started playing husngs and 6-max sngs in addition to ssmtts, which were always my bread-and-butter.

In these games, you obviously have a much more specific subset of players that play them (American-facing site), so you see a lot of the same bad regs again and again.

What I'm wondering is, is there a way to conceptualize the approximate returns to note-taking on these players? What I mean by note-taking is spending some serious time sitting down with PT4, filtering for # of hands vs. particular Villains in these games, and replaying all hands vs. Villain while making detailed notes. Clearly this is going to increase my EV -- the question is, how much?

Does anyone have anecdotal evidence regarding this? Better yet, has anyone tried to model their win-rate after investing time in making notes vs. their win-rate before doing so?

Anecdotal evidence? No.
But i do know the vast majority of your winrate in 6max sngs is gonna come from fish rather than regs, since there chips are easier to get and they are more likely to make major ICM mistakes. Recognizing and targeting common leaks and tendencies (ie. playing unbalanced) amongst the general fish population could be just as/more profitable.

HUsng's is obviously a totally different beast. Reg-to-fish ratio is probably gonna be important in determining how much of your time it's worth.

Depends on what kind of notes you make. Does he checkfold flops way too frequenctly? Then that is a note that is really going to boost your winrate against him. Are you making a note that he checkshoves river too infrequent in a 3bet pot when bet flop and checked turn? Yeah that's not adding anything really.

Very interesting question, sub'd.

I'd order them so:

HUSNGs - notes are key
STTs (6) - important
ssMTTs - relatively important

Pretty rudimentary, but I assume notes give you a bigger edge in HU and smaller edge in small stakes MTTs, due to the enormous fields.

Would be a nice experiment indeed, curious to see results myself.

The success/popularity of rather expensive heads up-specific HUDs and Notecaddy packages (not to mention the furore about the mysterious voice-activated "software aid" being discussed in NVG) would seem to indicate that the answer is "a lot".

Make sure you know roughly what a balanced strategy looks like, then take a quick note whenever someone does something significantly unbalanced. This shouldn't take long.

taking notes is great, getting notecaddy is better