I think the results would show that the accuracy of the GPS measurement, besides the obvious starting and stopping of recording the track at the precise Start and Finish lines, depends on the course.
If a course runs downtown, where there are skyscrapers around, the accuracy is greatly impacted. Same with running under a canopy of heavy tree cover. If there are many sharp turns, or turnaround points, that reduces accuracy, also.
I would love to get results from many units used in the same race, but apparently, many people can't download a track from their unit. I use a mapping unit, so that is not convenient to carry while racing (I no longer race). So, I will try to occasional solicit track files from an event, but I am not holding my breath.