My 5,1 runs like a brand new computer, especially since the update that got rid of the blinking screen. No plans to get a new computer, but if I did, I would get another Pro.
No it isn't. The ACD and Thunderbolt Display are both 2560x1440. Native 4K is over 3800 pixels horizontally. There are no consumer monitors capable of displaying that resolution.The Apple Cinema Display is. Not all of the details though. But it sure is better than 1080p.