For now, only "52nd Street" and "The Stranger" have been re-released by Mobile Fidelity.
If you don't have the original Sony SACD versions, I'd say definitely pick up the MoFi SACDs . There are definitely some sonic improvements on the MoFi versions over the Sony versions, in particular the smoothness of the sound quality. However, if you do have these titles on Sony SACDs, then it becomes an issue how big of Billy Joel fan you are.
Btw, the original Sony SACDs do have a 2 channel hi-rez mix as well.