I remembered that the first Unity was indeed built on Mutter, but later ported to Compiz due to poor performance. I also know Canonical practically incorporated Compiz to work closely for future Unity, so this is getting less likely.
But Compiz just seems pretty outdated now that GNOME3/GTK3/Mutter is becoming more mainstreamed, and it is known to deliver some performance issue, but on the other hand Mutter seems pretty good and is still steadily developing now, I'm just wondering if anyone related to the project is still testing and evaluating the possibility of Unity on Mutter? Not that you have to tell me now if you're going to do it or not. I just wanna know if anyone is considering it. Thanks.