Update: XL fit me more comfortably. 2X was more baggy than I'm comfortable with, so I think it's all a matter of YMMV. I do agree that you should try at least one size larger than your usual one with these shirts.
I also snagged a Mario bag and a Mario hat. I'm pretty content with my haul.
I could be wrong, but as I was making my purchase, I overheard a TM saying the shirts could run out by Friday.