If you're happy with your current monitor I'd say stick with it. I've a 28" 1080P 144hz myself and I sit about 2 ft from it. I can't see any pixels and the image looks decent to me. The only downer perhaps is the lack of HDR but I'll hang on for now also - there are a number of HDR monitors on the way and hopefully prices will come down ;)
No, not if you're happy with your current monitor. All comes down to personal preference. If you're not noticing the individual pixels then it really won't make too much difference, and you'll be more annoyed about the 3 inch loss to the screen. Lots of people have 1080p 27" monitors and are happy with them, all comes down to personal preference.
I've also heard this. Depends how close you sit to the screen though. If you're close a 24" will be better.
Would if make sense for me to sell my 27 inch 144hz 1080p monitor for a 24 inch 144hz one like this one? I've heard that 27 inches is a bit big for 1080 and I would get much better graphical fidelity at 1080p with a 24 incher.