Actually metacity also has compositing support so please test again WITH COMPOSITING ENABLED to make it fair. And please expand the comparative by throwing non-composited WMs such as WindowMaker, Fluxbox, E17, etc.
this would fix the performance issue in any compositing window manager, however, your pop up notifications may lack a shadow.
I think that if it doesn't yet exist, there should be an API for each window manager or a generic one that lets the 'game' or other fullscreen app preference itself that it wants to be unredirected for max performance. That way by default everything can remain composited but it can be easily changed on a per app basis based on preference, Ie unredirect for the latest 3d game, but leave on for things such as google earth
e17 hasn't just been a window manager for a loooong time; It's a desktop shell, much like Plasma and it has a complete Xcb widget toolkit, desktop widgets and all sorts of awesome libs for stuff like animation. It's more like KDE4 but without apps and it has a login manager complete with animated backgrounds, fancy desktop rain, snow, fire and a Xine frontend... You name it.
My experience with windowmanager, points to the fact that openbox, is the fastest, with regards to 3d-performance. Then the standard windowmananger in KDE is actually quite fast, and nearly as fast as openbox. And, particulary with regards to jitter, gnome with metacity, takes quite a performance drop.