I had the same problem and by chance discovered that my monitor settings override my GFX card settings so I changed the aspect from 'Full' to 'Aspect' within the monitor settings and voilà! no more stretched games. Might be worth checking out. Good luck :)

For me it was different. I had to reinstall an older version of my graphic card drivers (Crimson 17.4.4 to 17.2.1). A lot of people reported this issue on AMD's forum. Then i'd an other problem. When i ran the game, it was running in always centered even if i checked the option to just keep aspect ratio. I dont have this problem with PST. I found a solution to this : i disable 3D acceleration. Now, the game displays as expected, but ive got flickering cutscenes.
Any idea ?

The only thing I can think of that might help is forcing the game to run at your monitor refresh rate. I'm not sure if you can do that via the AMD software (I've got a nVidia card) but I use 'MSI Afterburner' which has an option to force/limit the framerate for individual games. It might help but as it is only cutscenes that are affected it might not. Might be worth a go though.

I finally found a solution for my issue. I can now enable 3D acceleration to avoid the fickering cutscenes. The problem was a option enabled by default in my graphic card driver : the "surface format optimization". Now, the game fits perfectly in my screen keeping the proper aspect ratio (before it was a little box in the middle).
Problem solved for me. I hope that could help somebody.