Hi Rowland,
Beyond taking a census of all users I'm not really sure how you'd get a thorough answer to that and even then what you'd want to do with any missing models. If you were talking of Canon & Nikon, I'd say list every model. They are virtually all used /mentioned as used.
Take a look at the drop menu on the right hand side of the screen on this camera sensor dynamic range comparison webpage ....
http://www.photonstophotos.net/Charts/PDR.htm
Best
JA
PS: Some models are duplicated because the sensor is tested in Full-Frame and Cropped mode where the camera has this feature on-board. I'm not sure how that would affect your data.