A well optimized game wouldn't need the top of the line GPU to run at the settings you described. I am happy I found the settings I could live with and achieve 60 fps but this game has SERIOUS optimization issues.
Please don't give Rockstar a pass and just call it future proof.
I have most things on ultra or high, so it really doesn't seem *that* bad from my perspective... but that's what has me so confused. Since my performance has been relatively good, I would have assumed it would have been good enough on a much wider range of systems... which doesn't seem to be the case. The dropoff seems to be sudden and kinda severe. I'm sure they'll get it sorted out in time.