I have come across players in some matches that have seemingly pulled off some kills against me that look impossible, but then seen them later in the same match and they have missed what I thought would be really easy shots to get against me, like when I am reloading.
The few that I have watched playbacks for can be put down to pre-aiming the corners, peeper's advantage, or sometimes where I have just misjudged my aim and left myself open.
I think camping is a bigger issue then cheating, but I've been guilty of camping in rooms for short bursts to recover energy or cover an objective, never for the entire match though, I do like to run around the map when I play.
I don't think cheating is anywhere near as widespread as you make it sound, I think I have only left 1 lobby where I thought someone was cheating, but I recorded the match and when I watched the playback I couldn't see anything that looked like cheating to me.