I got 14,000 miles off my first set of Metzler. They were stock on the bike and I think they were the Z4s. I replaced them with another set of Z4's.
The techs at the shop gave me a lot of grief because the tires were so square they stood up by themselves.
I replaced the next set at 11,000 because I still had tread but I was going to Midwest Track day and didn't want to get in trouble at the tech inspection. I had the tech at the shop look at them and he said they probally would pass for a novice rider but I decided I didn't want to take a chance and have to buy a set at the track.
We installed the new Z6's and I am impressed. I did not drop my tire pressure to 30/30 for the track but ran them at full pressure. I got to try them in the rain because our first two sessions were on a wet track and they stick, I was able to keep up with our instructor and had to grab brakes several times to keep from overrunning him in a couple of the curves. These tires do perform as advertised.
The other thing I really noticed about the Z6 tire is that when riding on concrete road surfaces the tires do not transmit the buzz/vibration that the Z4 tire did. The source of the vibration is the rain grooves/rough texture that is formed in the road surface to aid traction. I also noticed minimal tracking when we rode over the metal portion of a drawbridge. I remember some of these bridges in the past really messed with control of the bike.
If I can get the mileage out of the Z6 that I am getting out of the Z4 I will be real happy. The Z4's with 11,000 still have tread and are hanging in the garage in case someone needs a tire on short notice.