It's hard to judge the physics without jumping into the a real life counterpart of the car in question, but IMO all they did was increase tire grip to more realistic levels. N1 is your average all season tires, while N3 is average max performance summer tire. S tires are barely DOT legal tires, and R tires are soft multi-compound slicks. Almost no production car will come with anything greater than N3 equivalent tires. So if you want to judge a production car like the M3 or F430 don't do it on anything greater than N3. This is just my opinion, but I'm also going by the tire descriptions PD used in GT4.