I have been considering buying a large 21:9 monitor for a while now.
I decided to give this frontier forum post a go and see how the game plays at 21:9 aspect.
I did everything in the thread to get my desktop to 1980:810, loaded the game and set the res and windowed mode, then exited the game. Put my desktop back to its standard res (at 1980:1200 I had two large black bars above and below), loaded the game and it started at 1980:810 or 21:9... so all good.
If anyone is considering 21:9, I suggest you try out the tip on the frontier forums, it is very easy and you don't need to keep you desktop at that res to play the game with 21:9 aspect.
I also streamed it directly to YouTube and you can see how a cobra looks, playing at 21:9