Several other people had the same problem so I'll explain the solution in my case.
It was simply the Logitech G510 software suite (G15 using the same one now I guess).
You need to disable "enhanced graphics" in the logitech software.
When you enable/disable SLI, nvidia asks you to close the logitech software first if "enhanced graphics" is enabled.
It won't ask you anymore when disabled and no more 10FPS or such in WoT
Edit: just kill the Logitech software or find an old version maybe... Even without "enhanced graphics" it went back to 10FPS now: Alt-tab to windows, killed G15 software, Alt-Tab and 60+ FPS...
Edited by Donway, 10 May 2012 - 02:10 PM.