Well this is interesting- someone on the setlist.fm posted this when it was still the set I posted here-
That's has the same set as above, but someone edited it to the Countdown played in full set and said that was played instead. However, Megadeth's own tour dates section says that this Sept 20 show is Peace Sells and and that the previous night was a Countdown show. Very, very weird.
Also, they're definitely doing Devil's Island at the Peace Sells shows- tuning down seems to have really helped Dave's voice a ton!