Adrian

You can do what you like of course, but there are some things you can change and others you cannot. My attempting such changes just to try and make the door closing sound more to your tastes you may also create future problems for yourself.
The doors are already a water trap as they are in every Porsche model, but made a little worse in having no window frame.
Adding sound proofing may increase the door's ability to hold water, ruin your internal door trim and it may eventually start to stink. If the sound proofing starts to come apart it's likely to fall through the frame holes and end up jamming the window mechanism.
If you have leather trim on the doors it will even start to grow mold under the right conditions.
You would need to select your sound proofing material very carefully and don't use the Porsche stuff because that is the world's greatest sponge.
I also believe that you will not change anything by adding sound proofing anyway. Try draping a blanket over the inside of the door. This might give you a clue as to what you might be able to achieve.
The other choice is to sell the Porsche and purchase something with more suitable door closing sounds for your taste.
Ciao,
Adrian.

lanny

I just had my doors installed with Dynamat when they installed my Focal speakers and I actually do notice a minor decrease in noise- it is marginal however.