Rule No.1: never trust a dealer.
Rule No.2: see rule No.1

BTW: I forgot another "problem" with non-approved Porsche tires. Porsche could (theoretically) refuse warranty repairs for the 4WD system.
Porsche is one of the car companies who puts a lot of money, time and engineering skills into tire setup development and testing.
The problem with non-approved tires is not that they "don't feel good". The problem is in certain situations, especially regarding extreme driving (track, rain, snow, etc.) and braking. Take the Carrera GT as an example: with "standard" tires in the same OEM size, this car would probably be undriveable for an amateur.

I remember when I first heard about N-classified tires and I asked about that "stupid" N and the number behind it at my local tire dealer who is the biggest tire dealer in my town. The sales guy, somebody I always trusted, told me that this is just pure advertisement/marketing and that N-classified tires are the same as the tires without N classification. I trusted him and bought a set of non-N tires, same brand and type as the OEM tires for my 993.
A few days later, I had the impression that the car was a bit more difficult to drive at the (my) limit. After doing some investigation, I ended up with somebody from Pirelli Germany. This guy told me that car manufacturers keep the changes secret but there is definetely a different compound/tread mixture and other differences. And the "funniest" part: the tires I had mounted on my 993 were approved up to a speed of 272 kph only, the approved Porsche tires with N-classification were approved up to 290 or 293 kph, I don't remember anymore exactly. Of course my 993 didn't do 290 kph but just as an example of the difference.

I also saw that Mercedes started to do the same with their high performance car models, my former E55 AMG had M0 designated tires.