I'm still using a GTX260 and eventually figured out that apparently, for whatever reason, Depth of Field on High was what making the performance go below 60 (below 30 for cut-scenes). It wasn't until the cut-scene of mission 2 was when everything dipped below 30 and now, at mission 3 I'm pretty much playing the game at the lowest settings. Yeah, even I seem to notice some flat textures at the highest setting but to be honest, the game doesn't appear that bad like most games with low setting would so I have to accede MachineGames for not making everything look like flat shit (From the last-gen pics I've seen, even at the lowest setting, the PC version still looks better).
I am getting graphical glitches though, fortunately, they are not a regular occurrence. Mostly just hands or NPCs being stuttering, cut-scenes have flashes of random colors on the NPCs, one point, Deathhead didn't lip-sync with his voice, and even noticed the fabled white-texture at a distance in mission 1. Anybody figured out how to deal with those?