From 1080p to 900p you gain nothing cause there is no optimization. It really wouldn't matter.
This is a similar case toe Metro: 2033. Game was horribly optimized, not even high-end GPU's could run at enthusiast settings and achieve a smooth 60FPS at 1080p. At the time I had a 6970, and lets just say I had to game at Medium just to play around 40-ish frames.
Metro: Last Light, an even better looking game, more open areas, tons of new graphical features, and guess what? Oh my, ran 10x better than 2033. That 6970 which struggled with 2033 now running High settings with 50 - 60 frames at 1080p.
There's a difference between an optimized game, and looking amazing (MGSV: GZ runs on fucking TOASTERS at high settings) and games which are horribly optimized, look amazing, but run like shit.