I exchanged emails with a tesla rep about the headroom implications of the panoramic roof option. Below is his feedback:

Headroom for the panoramic roof and hardtop will be very similar. The hardtop ingresses into the cabin roughly 15 millimeters more than the panoramic roof. Unless your head is already scraping the top, the difference will be negligible.  

William13 | 14 April, 2012

Well to be honest the head room even with the Pano roof is only average.

ddruz | 14 April, 2012

Interesting thread. 15 millimeters is 0.6 inches. That's not insignificant where rear headroom is tight already. Given the printed rear headroom spec of 36.6" for the standard roof, the pano roof should then make it 37.2" measured the same way. I'd be very interested in comments from folks who have had a chance to sit in both the standard roof and pano roof betas.

jarsnice | 23 April, 2012

simple solution, get smaller passengers....duh

NJS1207 | 23 April, 2012

I am 6'1" and I had adequate head room in both the front and rear seats of the beta at the New York store, which has the pano roof.