I have bought a used Porsche 911 from a Dealership floor. I'm in the U.S., so the experience might be somewhat different (I'm guessing it would be different from Dealership to Dealership, in either country). I talked the price of a six model years old Carrera coupe (11,100 miles on the odometer) down about five percent. It was easy enough that I believe they were expecting it. I paid cash for the car, which might have affected the transaction (good? bad? People will argue this).
It's my opinion that a 911 cabriolet might come down even more with bargaining. Porsche charges a lot more for new cabriolets, yet they sell privately for less than the coupes. I think a Dealership would initially price them more in accord with the factory's line.