Mmm, not sure if it is just some bizarre quirk in my set up, but SLI isn't enabled by default in the Witcher 2 game profile on my system...

In fact, I couldn't find a Witcher 2 entry in my nVidia "Manage 3D Settings" list at all.

So I added "witcher2.exe"; set SLI rendering to "Force alternate frame rendering 1";

. . . and voila : 50-100% improvement in frame rate.

Scenes that were hovering at just over 20fps, jumped to 30fps and mouse control was noticably smoother.
Scenes running at over 30fps jumped to near 60fps.
[I confirmed SLI status by using the "Show SLI Visual Indicator" from the "3D Settings" Menu in the nVidia Control Panel]

Why would SLI be disabled?
Is SLI actually causing problems for other people? Or has it been disabled all along and no one noticed?

Edit: My PC
Alienware M17x, Win7 64xUltimate, Quad core Extreme Q9300, 8GB Ram, SLI 2x GTX 280M