Since I believe the last update my in-game FPS is limited to around 150-155 when I am playing the game with my usual settings and in Full Screen mode.
Interestingly enough when I alt tab out of the game or leave focus the FPS actually increases to what I have set in-game (limit - 299) as expected.
This seems to be a bug introduced in the last update.
Expected result: 300 FPS in-game when in Full Desktop mode.