While at the Santana Row store, I asked what the car's interior roof would be like without the pano roof. The rep didn't know -- as far as he (or I) could recall, every Model S so far has had the pano roof!
The current interior has lots of vertical where the glass is, of course, and that will necessarily be lower by at least a few millimeters with the solid roof and headliner. But, the pano roof also requires thick channels along the right and left edge of the ceiling, which presumably won't be there. The part of the ceiling I really want to see without the pano roof is the area just behind the pano roof. Currently, there's a large ridge there, into which my head bumped as I sat upright in the rear. If that bump isn't there in the non-pano version, there will be more usable headroom without the pano roof.