If you're getting more than 4000 miles on these tires, you're really not using them close to their performance limits, and I can't imagine wanting MORE mileage out of them, because as they get older, they all lose performance in less than ideal (wet or changing) conditions.
I'll bet the Metzeler product is just dandy. I've never had less than satisfactory performance from any Metzeler product.
In this current crop of sport touring tires from Michelin, Metz, Pirelli, Avon and even Dunlop and Bridgestone (after they figured out the failings of the BT020 and BT021 fronts)... the differences in performance would be undetectable to 99% of riders. If you do use the tires at the edges of their limits, you find strengths and weaknesses with each.
I'm sure I'm not pushing the envelope as much as you and some others since there is limited access to twisties in this neck of the woods. The 9,000 miles is mostly slab commute to work and weekend trips and I have no major complaint with the performance of the Z6 but with the new products comming out
There are slight differences in feel and differences in price. I think Z6s and their twin brothers, Pirelli Diablo Stradas, are about the best piece of rubber out there. Killer feel. If you're getting 9000 miles on Z6s, I don't think you can do any better. Now that said, a *new* Z6 will feel like sex with a Victoria's Secret model compared to one with 9000 miles on it!