Thinking about this more, the closest analog to stereo rendering would seem to be games with a split-screen mode. Which, in my experience, often does entail a performance hit (though certainly not a generation-sized one).

My guess is that how much a second viewport affects performance is highly engine-dependent. Ironically, the one game I remember having the biggest delta from single-screen to split-screen is the Dreamcast version of Quake III: I distinctly recall the sharp drop in poly-count on the otherwise curvy rocket launcher.