Agree, oh mouse aiming friend, that any changes at all in your system, especially monitor or screen settings, can really, really mess up your aim. I know Skuzzy and others had given advice regarding changing FOV, and I did mess with it, but found just going back to 1080p on the 27" new monitor made things look close enough that my aim came back, or at least the kill rate and such did. Maybe I'll mess with FOV and higher resolution some offline later and see, I made it bigger, not smaller, so maybe going in the other direction, and giving less FOV may make targets large enough that I can switch back to the higher res and look at the pretty lights as well as hit stuff.
I'm using one of the new "G Sync" monitors, tried it with both 970 SLI and 980 cards, and it works fantastically for removing little graphics issues, those little hiccups/problems in rendering, are completely gone in all games, including AH. It really makes a difference, not sure it's worth the 200$ plus for the little piece of board/tech in the box, but it does actually have a positive effect.