and does not add much rigidity, as I said in my previous post the Boxster design was roadster from the start. The body/chassis design stiffness was accomlished both structurally and metalurgically.
While the addition of a roof on the Cayman does make the chassis stiffer and reduces the need for stiffening the stiff structure components are not easily done away with or replaced with something cheaper.
Different forge/press molds would be needed to if for instance alum. was substituted for high strength steel. The form/shape of the part would need to be changed because a shape/form that works for steel does not work well for alum. Also, because these parts are an integral part of the car they are welded to other parts to make a unified chassis. Obviously an alum. structure member is not going to weld very well to its neighboring steel structure members.
An exception? Body panels for one. Now it could be that some Cayman body panels are alum. I can't be sure but I seem to recall the front trunk of my 08 Cayman S was alum. and the newer models could have alum front trunk, doors, rear hatch.
But so could the Boxster. I mean a lighter car which delivers better fuel economy and lower CO2 emissions is always preferred.
Porsche would not leave any gains in these areas on the table if they could avoid it. Also, it cuts stocking/tooling costs to make these panels/body parts from one material for both models.
I believe the weights I quoted are for a car wet -- that is with proper levels of all vital fluids but without gasoline -- but sans a driver. While some sources tout ballast is necessary to obtain the best alignment results -- I've had my cars aligned both ways and I can't tell the difference -- I've never coma across any vehicle weights that included this ballast weight.
Regardless the weights just don't seem right to me do not seem logical though I do admit logical is often *not* the best criterion to use to judge the correctness of numbers.