This may help some people but to get really smooth performance on my setup (i5 4690K@4.5GHz, GTX1080, 4K TV) by far the most important thing has been switching to 30Hz refresh rate on the desktop before starting P3D and then running the sim unlimited frame rate with vsync and triple buffering on. With my settings tweaked to be 30+ (mostly all high or ultra but no shadow cast on autogen) it is super smooth, fantastic infact.
P.S. I use Hotkey Resolution Changer to add hotkeys to allow me to switch between 30/60Hz (Ctrl/Shift/3 and Ctrl/Shift/6 respectively)