I'm not sure what the tuned SLK 55 AMG and the Chopster (tuned Cayenne Turbo) have to do with the Cayman S but anyway: the Cayman S is still a mystery...somehow.
Most people hope its gonna be a 911 competitor but all infos I got actually show a different picture: a Boxster Coupe, nothing more and nothing less.
BUT: the Boxster is a pretty nice little sportscar, especially regarding handling performance. For a 280 HP car, the Boxster S performance is impressive but of course it is not even close to SLK 55 AMG straight line performance (0-200 kph/0-125 mph in 16.6 sec.).
The Cayman S is rumoured to have a 0-200 kph/0-125 mph acceleration performance around 20 seconds but not below.
This is still around 4 seconds away from the Carrera S for example.
To make a long story short: if you want a FUN car for twists and turns with fantastic handling capabilities and a very good traction, your wife should go for the Cayman S.
If you want a car with a very good straight line performance, a nice engine sound and a very good automatic tranny, the SLK 55 AMG it is. The SLK's steering feels a little bit numb, my wife owns a SLK 32 AMG right now and the SLK 55 AMG even feels less sporty. My wife gets a Boxster S now, especially since I have to drive this car too from time to time.
I don't think that comparing the Cayman S and the SLK 55 AMG is really possible. The SLK is a roadster, the Cayman isn't. The SLK has a V8 and an auto tranny, the Cayman S has manual. There are so many differences.
Don't get me wrong: the SLK 55 AMG is a very nice car for a woman and I would probably recommend it over the Cayman. But if your wife is a very sporty driver AND knows what she's doing at the same time (mine isn't neither one
), she won't be happy with the SLK.
Good luck.