On the advice of a board member here, I recently mounted an Avon ST rear. People on other boards have been claiming 8-10k miles out of this tire on ZX12's, which are tire destroyers . At first I was concerned about grip, but after a couple of runs through the curves, I'm well satisfied with the amount of grip offered by this tire. If the mileage claims prove true, I'll leave all the 4k mile tires for the racers.
PS, once you hit the wear bars, you have 500 - 1000 miles left, but your bike won't be nearly as enjoyable because the tire will be worn too far from it's original profile.