Hi, I’m surprised that nobody answered you? I suspect that the best thing to do is sell the 911 wheels and buy some wheels for your Cayman.
The offsets are quite different from 911 to Cayman and I doubt that those 10.5” wide will fit because you have to get the offset just right.
Of course, it all depends on what your plans are for the car? If street, and stock ride height, and stock negative camber, the top of the tires are going to be closer to the outer fender lip or quarter panel lip. If you lower the car, put in adjustable camber plates at the top shock mounts,, and therefore dial in more negative camber, now the INSIDE top of the tire is going to be close to hitting the springs/shocks or inside of bodywork in the rear. GT3 Lower Control Arms can be used to widen the track a little and move the bottom of the tires OUT. It all depends on what you are going for?
I hope this helps some? There should be tons of stuff to read about this issue on this forum? Good luck! Tony