I still miss a unity vision instead of being 'we really wan't feature globalpanels,zenity and indicators + X, Y Z'. It's also quite a lot that's still beta/alpha and undecided. Like 'lets just pick that program run dialog' because it's already there. It is going to be less coherent design.
Hopefully they have plan for when to say 'no'.
About performance in mutter. It seems quite a few of them have problems. It's likely an easier problem to solve afterwards, comparing to the task of designing an desktop UI. But performance is also a 'feature' that will leave people impressed or not, because it's the first they will see. Not the feel of workflow, which could take weeks to judge properly. It seems clutter is more abstraction and less performance.
The downside of this is less desktop polish for everyone, since we now have a third target (unity) that divides the heaps of important user testing into three pools instead of two.