Professional gaming at 800x600?
Some pros play at extremely low resolutions to achieve the highest possible frame rate. Furthermore some pros clam that they have a competitive advantage at a resolution of 800 by 600 pixels, since enemies will stand out more in the less detailed image.
In Call of Duty: Black Ops III, Rainbow Six Siege and Overwatch you don't really achieve anything by changing the resolution. Your field-of-view remains exactly the same and in the case of Overwatch you even get some black bars. Dota 2 and CS: Go have a wider field-of-view at Full HD, which means that a higher resolution really only comes with advantages.