1) Yes and no.
Yes: It's usually not a problem of the GPU chip itself. But the choice of connectors can sometimes become the limiting factor.
No: All LCD monitors have fixed physical pixels, thus they have a "native resolution". You can't go above or below that without scaling or cropping. And usually it's indeed the monitor which becomes the bottle neck.
Extra note: Your R9 390 will probably have HDMI, DL-DVI and/or DisplayPort. Check your manual or some online reviews for the resolutions (and refresh rates) that your particular GPU supports.
2) Depends on your own viewing distance and eye sight. Or are you asking about the aspect ratio? Difference between 16:9 and 21:9 is VERY noticeable, as the 21:9 is very wide, indeed. But for what it matters, I'm using a 42" HDTV (1920x1080) from a 1m distance, and that's enough for me. On the other hand, some people think 1920x1080 is too limiting for 24", and would prefer 2560x1440 (for whatever distance they view it from).
3) 27" for £200 means you will have to compromise either on quality or resolution. With those attributes, I would recommend the
BenQ GW2760HS for £170. Though there is probably also a successor for that model already, as that particular model is over two years old. But I have recommended it multiple times before, and I haven't heard anyone come back complaining.
Ps. Remember that your monitor is usually the longest lasting component in your system, and it will be the main component you'll be interacting with (looking at it ALL THE TIME). So if you're spending good money just on the graphics card, then please consider adjusting your monitor budget upwards, as well...