I run a USB 3 external HDD for my MBA for games like cod and it works perfectly fine...
Do you use Stream? If so, any issues with using it from an external drive? I'm debating on upgrading the 64gb ssd in my 2012 air or just using an external drive.
Also, do you play if the battery with the external drive? Curious what kind of hit the battery will take running a game and powering a USB drive at the same time.