can I compare what ADS-B traffic I 'see' in the cockpit, with what ADS-B traffic the Atom ground stations are seeing (which hopefully is everything)?
I addition to what Peter has said, comparing what an ATOM station sees and what you might see is not really a fair comparison. For instance, I might not see a low level aircraft south of the Isle Of Wight because the Downs get in the way of his transmission to my antenna. You, on the other hand might see him, as you nip along the South Coast at 1500'. You are now 'better' than my ATOM station.
Likewise, I have an 11dB antenna on top of my house, while you probably have a 5dB antenna in your aircraft. In theory I would see many more contacts than you and you would feel very disappointed (since I am now 'better' than you), if comparing numbers and distances, whereas, in reality, your aircraft is seeing all the ADSB transmitters within XX NMs of you and allowing you to avoid bumping into them.
I can see you're trying to compare your maximum with a theoretical maximum but I don't think using ATOM stations is the way to go and, quite frankly, if you're seeing all the stuff that's anywhere near you then it is a pretty pointless exercise anyway. It doesn't matter that you can't see BA 123 at 350nm away but I can.