You can turn on metacity compositing in unity-2d (or run it with mutter). This will make unity-2d use transparencies and windows and menus will have cool shadows.
It appears to me that the fault of unity's performance is 100% on compiz. When you replace with metacity or mutter and use unity-2d it gets fixed.
Edit: I am trying out two scripts to switch between Unity-2d and 'real' unity. My computer is fast so the performance drop of unity is not a big issue most of the time. Only when running powerful games and unity-2d although fast, has some weakneses (The global window buttons only work if you use Ambience or Radiance themes, else they fallback to some ugly little thingies. Also the menu bar "blinks" instead of fading when you put your mouse over, it is hard to notice at first (compiz unity's fade lasts little time) but after plenty of use, it becomes obvious that compiz unity's menu bar is a lot easier on the eyes.
The advantage of these scripts is that I won't have to close every single app before doing the switch.
These are the scripts.
- From compiz unity to unity-2d:
-From unity-2d to compiz unity:
metacity --replace &
compiz --replace &