Even still Counter Strike GO is not a resource intensive game. No one needs a top of the line PC to play that game at max settings. Also pretty much any tournament is not broadcasting from the players computer but have dedicated observers that will always run the game at max settings. Plenty of CS pros still use 4:3 aspect ratios, but their games would be broadcast in HD.

>There have been some investigation of settings and high vs. low generally doesn't give you an advantage or disadvantage either way.

Yes and no. In a game like Starcraft where cloaked units give a slight glimmer when moving, pros universally optimize settings to enhance this glimmer as much as possible, and low graphics is one of the settings they use. More so though pros just stick to what their used to regardless of objective performance. It's why so many CS pros still use 4:3 aspect ratio.