I've decided that 4xMSAA+TXAA+SweetFX Lumasharpen is the way to go for this game. Other AA options unfortunately don't work for getting rid of shimmering in small objects in the distance (such as grass) and for some reason they make all hair and beard look very jagged. 4K DSR does quite well for this game as an alternative to its AA options but it doesn't work with G-Sync + SLI so it's a no go for me.
With a G-Sync display the above settings at 1440p seem to be quite ok, only noticeable drops in the woods (ultra grass). I'm not in the "under 60 fps at all times is unplayable" camp at all because I don't really even notice the drops with G-Sync.
This is probably the first game where I've been bothered by aliasing so much.
I highly recommend people give this SweetFx preset a try:
http://sfx.thelazy.net/games/preset/3571/
I took it as a basis for my own settings, I'm still tweaking to get a bit more natural skin tones but it makes the GTA V world look really lush without being excessively saturated.