I've got a stiffy....

From press coverage in Road and Track at the time of the Cayman S’s launch:

“Compared with the Boxster, the big difference of course is that the Cayman is a closed car. Porsche engineers have retained the Boxster’s strong floor-pan without modification, with the result that the close car is only 11lb lighter that the comparable Boxster.

On the other hand the Caymans S's torsional stiffness is 2.5 times higher that the Boxster S (which ranks as one of the structurally stiffest contemporary open cars) and only 5% lower that the 997 series Carrera. The beam stiffness is also doubled compared with the roadster’s.”