I don't think being a civilian means one iota of anything in regard to being really ready and comparison against the cops doesn't either. Both groups have been to shown to have outstanding individuals that perform exceptionally well and both groups have been shown to have folks that are more of a threat to themselves, bystanders, and other possible victims more so than they are to the threat they are fighting.
You bring up some good points. I guess what I have seen it is hard to measure and to predict which individual or group will do well, and which will not in an actual shooting. I have seen a few basic studies in years past but I dont remember anything really special out of them.
One item I have noticed several instructors mention, and it was a factor in a local shooting. I will give two examples below:
1. A more outspoken, generally larger officer, who is good at the non-lethal aspects of wrestling, and restraining suspects, generally do poorer then their oppisites.
2. An officer which are smaller, and generally more quiet, and less apt to use any force unless absolutely required, generally do better in a shooting situation.
As I said I cant cite a study on the above 2 examples, but it has been mentioned by several instructors.