I think her performances in the last months have always been pretty much amazing. Of course, sometimes it was playback, but I noticed she takes pre-recordings of her best live performances for the appearances where she's not singing.
Notice, at X-Factor, she sang the whole song live except the last part where it gets very intense. She took the Billboard Awards recording and put it there, lypsincing on it for this part. I think it's not too bad, a lot of artists do that to give a break to their voices. As long as they "sometimes" perform live, I think it's okay.
Anyways, before it turns into a TL;DR, just wanna sum it up : she's one of the best live artist of the moment.