Although I wasn't in the 12/19 group, I also updated to 1/18 (actually 1/17 but it happened on 1/18?). I thought my numbers were pretty good for Penn, but maybe not? I submitted kind of late, didn't write a Why Penn, and also just got WLed at Chicago, so it's possible that my app sucks.
However, I thought this thread had concluded that the dates mean nothing? Maybe I'm thinking of another school.
I'm in the same situation (applied a bit late, 12/19 first update, 1/17 update on 1/18). Also thought my numbers were good for Penn, but I guess we'll see.