I also play on a 5k 27 iMac and I have noticed since the patch I can hear my video card fan spool up almost constantly now where as before it only did it in very limited situations, usually around lots of water. What are your settings? They defaulted mine to 5 and still the same thing running at 1080. I am also noticing a lot of video lag when moving around my characters with the mouse.
Not happy with this particular issue at all.