Finally figured out what's bottlenecking my 980 SLI experience: VRAM when driving and loading in new objects at a fast pace.
GTAV has got to be one of the first games that's really really limited by 4 gigs VRAM. Running at 1440p/1527p/~1600p or 4k eats up a ton of VRAM. So does MSAA/TXAA. Everything else hardly makes a difference besides distance scaling/population variety if you've got a 980 or higher.
Once I figured out that, I found a happy medium by dropping population variety to practically zero, freeing up extra VRAM bandwith for higher resolutions. I'm running at ~1600p with just FXAA and it's pretty smooth now when driving. Even at 1440p, throwing on some MSAA/TXAA is just too much for the VRAM, so it seems like it's better to try to get to 1600p DSR with some smoothing and FXAA.
When walking around on foot none of this really applies and 4k or 8x MSAA/TXAA is fine even with 4gig VRAM and it'll be the gpu/cpu power limiting the FPS there. Since objects load into view, much, much slower.
Man, I hope GTAV is one of the very, very few games in this PS4/X1 based generation that's highly limited at 4gig VRAM. It sucks if stuff like the 980 have the raw performance to push FPS and effects, but turn out to be lacking the VRAM to do 2k-4k with decent AA because of VRAM limits.