From the Porsche Cayman S Product Information booklet (section 12.7 Interior Dimensions):

Effective headroom, front (mm/in)
MY06 Cayman S = 959/37.8
MY06 Boxster S = 972/38.3
MY06 911 Carrera = 974/38.3 (w/o sliding roof), 966/38 (w. sliding roof)

I just pasted this directly from the document. No idea why they say 972mm = 38.3in for the Boxster S and 974mm = 38.3in for the 911 Carrera. I think the metric -> imperial conversion is flawed. The original source is at:

Cayman Product Information Guide Chapter 9-13 - sorry, not sure if you have to be a member to view

Being 6'2" I'm also interested in the fit test, hopefully the local dealer will have one to try soon.