Just recently upgraded from 1080p 24" to 1440p 27".
I'm in the same dilemma. I've adjusted most UI I could find, even while in duty (sorry, party-members). I'm glad how much is adjustable, especially after learning of CTRL+Home, but something like names, forming party, or messages/notifications are not adjustable. It's either 100% or 150%. I'm atm with 100%, because 150% is insanely huge. If I was still on 24", 100% would be way too small, but 150% still too large.

Request:
Please implement more steps between 100% and 150%. Just like with "Default UI Size" right below. It would solve a lot of problems.
And while you're at it, remove the "HD", "Full-HD", and "4K" naming. Considering the percentages, 4k being only a third larger than Full-HD doesn't make sense.