Kappa didn't exist when Solstice was a concept. Bob Lutz was focused on the LOOK, and to match whatever architecture necessary to preserve the LOOK. When it became apparent the Solstice LOOK could not be designed using existing architecture, Bob fought to develop the architecture necessary to preserve the LOOK. In other words, the Solstice wasn't engineered to fit the kappa. The kappa was engineered to fit the Solstice. Up to that point, it was mostly U.S.
But when it came to actually engineering the beef of the car (i.e., Powertrain), GM tapped their global engineering expertise, which included Opel, Saab, and a host of other engineers, including U.S. participants.
Basically, the design, looks, and the basis for the kappa architecture we're mainly U.S. driven. Everything beyond that was largely a global effort across GM.
2008 Redline Auto