CP,
The cars are so new, I'm not sure anyone really knows how much room is under the fenders yet. You might look at
www.wheelenhancement.com and call them and ask. They're in the LA area, but ship anywhere. These guys really know their stuff about Porsches and the wheels and tires that go on them, but I'm not sure if thay'll know much about what fits on the Cayman yet.
brad