Eh, I talked to her in the 2nd or 3rd inning and she said it wasn't loud at all. She did text me back when they started rallying to say "it's loud now." She had a blast and said it was great, but similar to what Ag07 posted, she said they got really loud when they did something, but other than that MMP was louder, I've never been for a playoff game, actually never been to the new stadium so wouldn't know.
As far as the capacity, just going off what she said and what I saw on TV. Also, when looking for tickets for her I actually went to the Yankee's website and they had basically a half section completely empty and available, so I don't think it was a sell out. I'm sure they aren't as excited as us for an ALCS, they reserve that for the WS, but I was still shocked that 1) the Yankees still had seats and 2) I got her dang good seats for under $125, those sameish seats at MMP were $250+ for games 1 and 2. It did probably help I was looking for a single, though.