If you have an SLI setup, disable SLI and start the game. It will resolve most of the performance issues since SLI support is non existent.
What is this fix?
This is a Nvidia Inspector tweak that makes SLI work in Witcher 2. Currently it doesn't. After this fix you should see noticeable FPS improvements w/SLI. For example, with SLI GTX 580s you'll see a considerable gain in FPS at Ultra Spec w/UberSampling enabled. That said, there may still be more room for improvement down the road in official driver/game updates.
None to my knowledge, however I've only tested this fix for my GTX 580 TRI SLI; if this fix doesn't work for your other cards then maybe it's card specific? If you somehow mess up in Nvidia Inspector, at worse you may need to reinstall your Nvidia drivers.
I've heard of a "light leaking" bug with SLI, that is possibly fixed by forcing AFR2 (Alternate Frame Rendering 2) for The Witcher 2 (either through Nvidia Inspector or the Nvidia Control Panel). I haven't seen that bug yet.
This fix has been tested to work for Nvidia drivers 270.61 (stable) and 275.27 (beta). There is no FPS improvement between the two after much testing. The beta drivers however have improved 3D Vision compatibility for The Witcher 2, and maybe other non-documented tweaks.
1. Download and install Nvidia Inspector. (latest version to my knowledge is 220.127.116.11)
2. Open Nvidia Inspector. Click on the tool button at the right of the "Driver Version" box. This opens up the Profile Settings editor.
3. In the "Profiles" drop-down select "The Witcher 2". Under "Compatibility" (first section) look for "SLI compatibility bits". You want to change this field's value from whatever it is to 0x02C04205.
4. Hit "Apply Changes". You're done! There's no need to change anything else.