Unity seems to beat Gnome Shell in startup time. Everything's up and running in half the time compared to Gnome Shell. But then again this isn't where we'll be spending most of our time.
It all boils down to how frequently you reboot your machine. If you're testing the development version of Ubuntu, I expect that's quite frequent.
Unigine seems quite a popular benchmark tool these days. Not surprising considering it's one of those few demos that can not only stress your GPU, but also supports Linux.
There's no significant difference in OpenGL performance whether you use Gnome Shell, or Unity.
Unigine Tropics only has a 1fps gap advantage to Gnome Shell. That's hardly anything to write home about.
Unigine Heaven, again, doesn't show anything significant.
Of course, it's not that there isn't any performance drop. It's just that Unity isn't necessarily the culprit here. A more thorough testing would probably be needed, but it seems something lower in the graphics stack is giving problems here.
Subjectively, I can definitely perceive a drop in performance as each new Ubuntu version rolls out. Even Gnome Shell is showing slowdowns in Ubuntu 12.10 where it wasn't so bad in older Ubuntu releases.