I agree you hit memory limits per application if you run a lot at once, but WindowServer roughly uses about 10-20% more memory on 4-5k vs. 2k, running the same

I agree you hit memory limits per application if you run a lot at once, but WindowServer roughly uses about 10-20% more memory on 4-5k vs. 2k, running the same applications. The 1080 plus the external, perhaps 15-25%. I'm not saying this means 8GB is the right amount of RAM for you.