I dont know much about 3d but I think the fps should be no different from 2d. All your changing is the way you view the game. I may be wrong.
i honestly dont know about performance, however I do know that the witcher 2 is a hog on graphics.
Also wen in 2D make sure you enable VSync with higher end cards, they tend to overwork themselves and produce 100 - 200 fps when actually you only need around 30 - 60
witcher 2, would I be able to get a decent frame rate with 3d on high/ultra settings.