Jamie McWhir - technical manager at Michelin and prior FI tyre technician for them - explained to me how "N" marked tyres differ from the generic tyre. You could have a generic PS4S and "N0" PS4S but they aren't the same. The Porsche marked tyre has been developed for and with Porsche to their specific requirements and tested to ensure it meets their requirements.
Or... he's telling me a load of old bull?
But then Dave Mullin at Goodyear and Sam Taylor at Pirelli have explained exactly the same to me, including the difference in physical dimensions, i.e. a 235 wide generic tyre may be wider (or narrower) than a 235 "N" marked tyre as the latter is made to a more precise spec whereas generic tyres are allowed to be within wider tolerances.
I spoke to Dave years ago when I had my Audi TTS...
He told me all tyres are made in accordance with the ETRTO rules: http://www.etrto.org/page.asp?id=1690&langue=EN
This means a generic non-AO 255 35 19 tyre is allowed to be up to around 270mm wide, and is also the reason he receives dozens of calls from Audi owners about wheel arch rubbing on some models, as owners have bought generic tyres and not AO marked ones.
AO tyres are manufactured to Audi's own spec to meet their performance, noise, and comfort criteria and also so as to fit within the wheel arches! So a 255 wide tyre can be up to 4% narrower to accommodate this AND still be within the ETRTO rules. With AO tyres he doesn't get calls about wheel arch rubbing.
So, apart from warranty considerations, there are some other real reasons to stick with "N" marked tyres perhaps?