ResB:
I spoke to UK insurance companies about the N Rating. They specifically stated, using non rated Porsche tyres would not invalidate your insurance.
In the UK based Porsche enthusiast circle, the consensus is (perhaps the cynics) that N rated tyres is just another Porsche money making scheme as the comparable non rated Michelin for example (MPS2) is about £100 a tyre cheaper yet the technology is 10 years or so newer!!
In Europe, tire brand and type can't be "forced" upon customers. Meaning: even if the manufacturer states a certain tire/brand and N-rating in the car documents, you are free to use whatever tire brand/type you want as long as the speed rating and size is what the manufacturer asks for. This is a very dangerous situation in my opinion but the EU calls this free market. 
So it is no wonder that insurances don't care, they can't care because it would be illegal.
The "money making" scheme story is just dumb but when an urban legend goes out, it sticks for a very long time. I'm sometimes very surprised that Porsche enthusiasts all over the world pay 100000 EUR and more for their car but try to save 50 EUR on tires. Tires and brakes are in my opinion the most important parts of the car, the tires actually connect the car to the ground. What can be more important ?
In Germany, the price difference between N-rated and "standard" tires is pretty small. Last time I checked it was only 30-50 EUR per tire.
Btw: since Porsche does not sell these tires, i wonder why people would think that Porsche wants to rip them off with the N-rating ? 
--
RC (Germany) - Rennteam Editor Porsche 997 Turbo, BMW X5 M, BMW M3 Cab DKG, Mini Cooper S JCW
