Do any of the three perform better than the others, in the games they all mutually support? (like Deus Ex Human Revolution) The Nvidia drivers take a huge hit - typically to half framerate running 3d modes, and if its combined with multimonitors can be even worse. I was wondering if they are all like that or if it's just inefficient code..

It depends on the game and hardware used. It wouldn't be wrong to assume that nvidia performs better than iz3d but i don't think the difference is big. Tridef however takes quite a hit (remember i have an ancient machine) performancewise but it has hands down the best visual quality (least ingame anomalies) of the 3. One thing to take into account is that iz3d doesn't seem to be updated anymore even if the software still plays decent with several games.

All the drivers take about a 50% hit (more or less depending on the game and the setup). Personally I feel Nvidia might have a slight advantage, but they are all very similar.

Here's a benchmark I did with Resident Evil 5:
viewtopic.php?f=3&t=4469

And here is Street Fighter 4:
viewtopic.php?f=3&t=4479

As far as I could find out, Nvidia works far better with SLI.
You nearly don't take a hit if you are going stereoscopic.
I even heard of higher framerates in stereo than in 2D on an SLI setup.

DDD unfortunately doesn't take such big advantage from SLI.

Ive been using the Tridef DDD and a dual DLP projector 3D setup for gaming and have not had any issues. playing on a 9' screen in passive 3D, no lag looks amazing. using a single Nvidia Quadro FX card to drive the dual projector stack.