Ok so I bought the game on Origin just to eliminate the possibility of the version being bad, it cost me 3€ so no big deal.
I did what you said in the instructions, here's what my flawless widescreen looks like:
I also disabled scaling in my Nvidia Control panel:
I started my Mirror's Edge game and set the resolution in game to 2560x1080. Then I run Flawless Widescreen with the Mirror's Edge plugin enabled. When I'm done with that all the menus look low-res, this doesn't bother me, it wouldn't be a problem as long as the game actually worked in 21:9.
However when I start a new game, the game is still in 16:9 with black bars on the side.
Here's what my Mirrors Edge plug-in in looks like.
Oh and when I start the game it briefly flashes to full-screen and then immediately goes back to 16:9
I'm on Windows 10 64-bit with a GTX 970.
Anything else I should try? Need any other info?