A bit of the physical difficulties were touched on in the "Clouds of Glory" thread you participated in.
A pretty good assessment of gaming priorities was given by Al (Chock) in the "effective range ..." thread (
http://www.theaerodrome.com/forum/470525-post3.html) -- from which I took that drawing any conclusions concerning what it was really like from a video game carries with it a fair amount of risk.
As to Dave's comment: Since he had nothing else to do (except maybe not fall out of the airplane?) a gunner could give his full attention to the task at hand --such concentration would, I assume, pay dividends in accuracy.
An essential ingredient would be pilot/gunner coordination: having the pilot throw the airplane around just as you had a target lined up would not help. I understand that those aircraft that positioned the crewmen close together (allowing for more effective communication) were much more dangerous foes to tackle. A crew that had flown together for many missions would also, I would think, develop a symbiotic relationship in their respective jobs: the pilot knows when and how to position the aircraft for a good shot, and the gunner knows when evasive maneuvers are at hand (and that it's time to grab on!).