[70Sixter]

Porsche believes that tires are too important to their cars' performance to not have a say in the tires. You'll notice in the tire TSB that the manufacturers do not have a blanket N rating for all their tires.

That said, the manufacturers that do not work with Porsche on their tires could well have as good or better tires than those that do and that have N ratings.

The ratings change periodically and some drop off the list while others are updated as N1, N2, etc.

The design of a tire for a front wheel drive/front weight biased car could logically not work as well as for a rear drive/rear weight biased car.

You also must have a tire rated for the top speed of your vehicle, at least at time of manufacture.

Tire Rack is a good place to do research even if you buy elsewhere. Both their own testing & ratings and user reviews.