the $20,000 per set price is for the GT2 long distance brakes... I doubt that the regular 996 TT brakes would even come close to that figure... probably a little less than $10,000 for both front and rear for the same brakes to be put on the M5....
Unless you are driving on the track regularly, I don't think you will notice a difference. Might even be worse, because I bet the brakes take a little while to heat up. Racing GT2 conditions are far different than street and the occasional track event.
I don't know. The ability to stop on a *freaking* dime regularly with no fade and basically no wear on the system sounds extremely appealing to me. I'd certainly dip a little deeper into the engine's lovely power reserves more frequently.
Also, less wear means not as much brake dust to deal with. I really like clean wheels