I bought the game from Steam and when I start it, it first has the windows taskbar visible. When I alt+enter, I get a small windowed version of JC2 (as expected). But when i alt + enter back, I get a full res version in windowed mode.
The game also appears to boot in 1920x1053 instead of 1920x1080.

What I tried so far:
Change resolution ingame to whatever possible
Disable/enable fullscreen ingame
Run in Win7 compatibility
Run in Win Vista (SP2) compatibility
Delete JC2 settings file.
Run from steam
Run directly from C:\Program Files (x86)\Steam\SteamApps\common\just cause 2

Any help would be appreciated.

My setup:
Win 8 Pro
Sapphire Radeon HD 6850 Toxic
2 monitors (JC running on main monitor at 1920x1080, second running 1680x1050)