Maybe Franc is speaking about the "original" chassis design that came from the SLK.
I do dissagree with the body stiffness statement.
There has been a XF roadster sitting in our garage all summer, and I've driven it several times with the top up and down, and the chassis stiffness doesn't compare to my coupe.
I think the XF roadster makes one of the best looking roadsters out there (with the top up). But I've always been a coupe man when it comes to looks.

XKE coupe
63/67 Corvette Coupe
Viper coupe
Porsche (any model) coupe
Ferrari (any model) coupe
Mercedes SL (Gull Wing)
ETC.