As far as I know, there are plans to port Unity from Mutter to Compiz due to some performance problems caused by the first.
If these problems with Mutter are fixed at some point in the future, and considering that stock Gnome uses Mutter, would you consider porting back to it? Would it still be technically possible?
"Plans to port" meaning Unity has been ported to Compiz, and we are just smoothing out some rough edges before we release a Compiz based Unity.
While it would of course be technically possible, it would be a lot of work for no considerable benefit. The time that would be spent porting Unity BACK to Mutter could (and definitely will) be spent making Unity rock beyond understanding.
The animations will still be there of course, and we hope better. The thing about the port is that we switched away from Clutter, because we were reaching technical limits. And this is not a lighthearted decision, considering what we've invested in our previous Clutter-based implementations and also considering that our lead developer (Neil Patel) worked on early versions of Clutter and still has a lot of respect for the work of the Clutter team.