reginos:
I understand that N rated tyres have some differences (compound, construction etc)compared to the normal spec in order to suit the Porsche characteristics, even though they look the same. The N rating is not just a seal of approval that a tyre is of a certain quality but it goes beyond that.
Especially in the US, where Porsche customers usually buy tires at independent tire shops and not at their Porsche dealer (cost factor), tire shops are telling Porsche customers that the N rating is just a scam. Which is absolutely wrong. Why? Well...tire shops, especially larger chains, usually buy tires in huge numbers, so they get a special deal. The non N-rated tires can be used for all kind of cars, not only for Porsche and sizes are sometimes very similar or even identical. In Germany, we have a similar problem with larger chains but they started to change and to admit that N rated tires are different. Finally.
In reality, the price difference between a N rated and non N-rated Porsche tire isn't huge, I encountered differences of 30-50 EUR per tire in Germany, which really isn't that much in my opinion. Especially the 911 with it's very special weight distribution, needs a good tire setup.
The N rating assures something very special: The tire XYZ with the N1 rating from 2009 is the same tire as the XYZ tire with the N1 rating from 2012. The N rating not only has a different compound/tread mixture/structure but also assures that the tire with the same N rating is always...the same. Tire manufacturers always "improve" their tires, so a Michelin PS2 from 2010 may not have the absolutely same compound/structure as the PS2 from 2012. Just another example.
There are two things people never should care about cost on a high performance car: The brakes and the tires. Unfortunately people spend 130k on a sports car but want to save 100 bucks on brakes or tires. 
More myths regarding tires (repairs, tire pressure, mounting):
1. (Punctured) tires can be repaired: Yes, they can. No, you shouldn't. Very dangerous. I've seen many patched high performance tires in the US and other countries, thankfully, this stopped in Germany. High performance tires with a speed index V and higher, should not be patched/repaired. NEVER.
2. Tire pressure: Manufacturer recommendations suck. Wrong. Especially Porsche is very accurate about tire pressure recommendations. You DO NOT lower tire pressure for track racing, this is something people kind of got wrong. You need to drive a warm-up round, then check your tire pressure and reduce it to manufacturer recommendation (tire pressure is higher after warm-up round). Do not forget to correct the tire pressure after track racing (after the car and tires have cooled down...the tire pressure will be too low).
Adding MORE tire pressure than recommended doesn't help either. Unless you need to raise tire pressure because of maximum load or high speed driving (manufacturers usually have recommendations for that in the manual or in the car). Adding more tire pressure (more than recommended) doesn't improve driving stability, on the contrary. The car can get very tricky at the limit and you can loose control much easier. Keep in mind that with raising tire pressure, the surface of the tires actually touching the ground, is reduced.
3. Mounting (for example taking off summer tires, exchanging them with winter tires and putting the summer tires back on later on): Tires with a size larger than 18'' are very sensitive to mounting. When removing the tire from the rim, the tire can be damaged. This damage is usually invisible. NOT recommended. Same with track tires. Always keep a second pair of rims for winter and/or track tires, do not use the same rims for all tires you want to use. Dangerous.
I know that some of you will not agree with me but this is not something I just sucked out from my own phantasy. 
Agree or not...it is YOUR life and your car you're toying with. 
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RC (Germany) - Rennteam Editor Porsche Panamera Turbo S (at Porsche right now), BMW X5M, Mercedes C63 AMG Coupe PP/DP, Mini Cooper S Countryman All4