If u have PASM, there is no problem in getting 19" whatsoever.
The ride comfort will be almost the same...and so will the handling.
In fact, the 18" are probably better handling than the 19" - but 19" look better.
If u DONT have PASM, think twice, cause it may be a hard ride, indeed.

The reasons why Porsche doesn't officially deliver 19" to the former 986S model, are these:
a) Politics. "Let's give people who buy a new model (987) something extra".
b) Ride comfort and durability: The 986S (no PASM as u know) is hard to begin with, and 19" rims will kill any comfort left in the car + in the long run possibly shake and rattle the car apart.
U can easily fit 987-rims on a 986.

So PASM or no PASM should really decide for u IMO