I'm still debating whether I need a GPU or not..
Editing the amount of RAM dedicated to your video card may help glitching. I haven't experienced any glitching yet.
As far as turbo, how are you checking? Intel power gadget?
I think you have two options: 1) Make sure you have it generating p-states in Clover 2) Create a SSDT using ssdtPRGen. There are directions floating around for Coffee Lake.
I would indeed drive a 27" 4K display. ... I sense 'some' kind of lag during use but not sure whether it's CPU or GPU-related.