N-Spec refers to tires that have been tested and certified by Porsche. This is more than a rubber stamp (regardless of what some people on these forums will have you believe), Porsche actively tests the tires as well as analyzing the specification to make sure they meet the engineer's requirements.
In application the "N" is followed by a number. That number increments if the manufacturer updates the tires or introduces a new tire line in that particular size. Porsche doesn't specify a specific N number by model but they do state that you shouldn't mix N numbers on the same car. That means is you have N0's on the front you can't put N1's on the rear. You should also never mix brands as the N1's from one manufacturer have no relation to N1's of another.
The major advantage in using N-Spec tires is to ensure that your car is performing within the limits it was designed to. Using non N-Spec tires isn't a problem but it can introduce handling and ride issues that your dealer (if the car is still under warranty) cannot resolve.
On the flip side, there are many Non N-Spec tires that people have used on their Porsche's with great success, Bridgestone S-03's are a case in point.
As to why there are no N-Spec PS2's in our sizes, I think that's primarily because by the time Michelin had the PS2's certified by Porsche our sizes were no longer a size that any current Porsche uses. I believe the tire makers only go through the specification process when there is an expectation (or contract) that they will be used a OEM. PS2's in our sizes will probably never be OEM so it's unlikely we will ever see an N-Spec PS2 in the 225/40x18 and 265/35x18 sizes.
BTW, many here have used the PS2's and love them. Many seem to be in the 17" size but some have also used them in the 18's as well. I tried the 18's and hated them but YMMV.
I didn't like them because their high slip angles cause the front to be numb and the rear to be twitchy. They also tramline very bad compared to the PS Rib (N1). On the plus side, the highway ride was excellent up to 90 mph where the soft sidewalls reared their ugly head and made the car wander a bit. Not something I like at 110
