I can render at 2800x1368 at 0.9 detail and 9 AAA on my friends TG3, his computer is a beast mind you. But I noticed I can render a image out in less then 12 hours with ray tracing out. With it active, my friend often has to fail my renders because they'd had been rendering a week or more.
So I think, that is your kicker: "Ray trace objects: Yes, Ray trace atmo: Yes, GI surface details: Yes" causing the most slowdown. Even on my demo version with it off sample renders run in minutes compared to hours.
If you turn it off, crank resolution a bit higher, and render, you can add some good effects via Photoshop and downscale and ray tracing wouldn't be visible if it were on or off. Least I've noticed not much a difference with it on our off in highly detailed scenes that have been downscaled. May be wrong though.