As good as the Boxster chassis is, it is no where close to the stiffness of the Cayman. It is amazing how much strength and stiffness that the roof structure adds.
Take a look at the Boxster chassis shown below - the middle of the car is basically the floor pan - that's it! In some ways, its amazing that the car doesn't simply fold in the middle. As such, there is only so much engineers can do to make the floor pan (a mostly flat panel) stiff.
The Cayman adds the roof which creates a very nice box-like structure to greatly improve chassis strength and stiffness.
A Boxster Spec race car achieves the same by using high strength steel tubes to build a box-like "cage" in the middle section of the car. This cage not only protects the driver in a crash, but adds substantial stiffness to the chassis.
