"Visible pixels" are a total non-issue already on a 1080p screen, and a near-non issue on 768p. There's just no ambiguity about this, it's a matter of simple physics. Maybe you'll need to go up to a 1200p screen or thereabouts to cope with crappy rendering on the software side (allowing for a 0.7x factor or so in image spatial bandwidth/resolution due to lack of proper anti-aliasing), but anything above that is just plain overkill. Unless you like to look at tiny portions of your screen with a frickin' magnifying glass, of course.
You can definitely see the resolution difference between a 1080p 13" display and a 13" 'retina' display. You may not care about it, but I think it's uncontroversial that it's a visible difference.
You can see pixels up close on a 1080p screen if you have good eyesight, but that's not the way you're supposed to work with a screen as a matter of ergonomics. Even on a laptop, you're always looking at the screen as a whole, not just seeing a tiny portion of it in your field of view.
That's exactly what I meant by "up close". A 27'' monitor should be 3 or 4 ft. away in order to comfortably look at the whole screen. Any other choice is terrible ergonomics.
