Joe's ep was the highest rated ep at the time, but later episodes bested it. By how much, I don't know. I still believe that many people tuned into the show in the first place, waiting to see his ep, and then stuck around after they got hooked. I wish he'd have made a return appearance, as I was never interested enough in the show again to watch.
Re watching SGU for McKay, I made a vow to never watch SGU and I'm going to keep it. But I'll be searching YouTube for his scenes the next day and looking to see if any mention is made of Atlantis and the rest of the team.
(Now if Sheppard was the one to appear, I may have had to break my vow and ended up in Purgatory. :O )