I don't know about other 21.9 users but I find that to really appreciate the extra FOV/wideness, you want to be sitting a bit further back than what you would with a somewhat equivalent 16.9 display and as a result this gives the perception of an even sharper looking display so 27" 4k is better for sharpness if you like to sit pretty close to the display. But again, this is entirely subjective, some people like to sit really close to big displays.
Regarding price, some youtuber asked about the pricing for the 27" model and the rest of the world and the asus rep said 2000 everywhere:
https://www.youtube.com/watch?v=6MCgM78Sg7s&feature=youtu.be&t=4m26s
If true and I am interpreting the asus's rep answer correctly, that is probably good for us given how UK generally gets screwed over for VAT, gouging etc. etc.
Worth taking with a pinch of salt as always though...
I think the main things to worry with these monitors will be:
- halo'ing around bright objects on dark backgrounds
- the trailing/delay for the zones to match what is actually happening on the display
- if a VA panel, trailing/smearing/ghosting
- if IPS, probably still backlight bleed issues but hopefully not as severe....
Regarding the last question, not exactly sure what you mean, you mean that if a game is only getting between 60-90fps and the monitor is set to 200HZ? Gsync will be operating between 30-200 fps so there shouldn't be any issue/stutter. Or do you mean there are games out there that don't go above those refresh rates? If so then no there shouldn't be any issues there either.
Of course if you want the full experience of high refresh rate gaming/200HZ then you want to be getting as close to that as possible.
EDIT:
Also, whilst it is 200HZ, hopefully the pixel response time will be able to keep up as other 200HZ screens (granted these are overclocked ones though...) don't really get much benefit due to the pixels not being able to keep up and as a result, it is better to settle with a lower refresh rate i.e. about 120HZ in the case of the following:
http://www.tftcentral.co.uk/reviews/acer_predator_z35.htm#response_times