The Cayman has been raved as a super-excellent handling car given its mid-engine.
I've read debates and comparisons between the 2 over and over on the Porsche forums.
Porsche will always keep their top of the line model 911 above the Cayman or Boxster.
Which means, the Cayman will never be as fast as the 911.
But IF both cars have the same amount of power, I'm sure the Cayman will have the advantage over the 911 on the track.
The Cayman is a 2-seater, while the Carrera has "rear seats"...err...yeah, only for toddlers!
You're also right about the C4S being heavier due to the AWD, and that taking away it's launch time. It is a little slower than the C2S, but then again, with 320 German horses, it's not really sluggish at all! The good points of the C4S is the looks, it has the same wide-body, suspension and brakes as the Turbo model. And it's supposed to handle really well in the wet, and snow. Though I haven't driven mine in rain yet! When it rains, I drive my Chevy S10....I know, I know!