Without looking at the offered sizes for the Z1, do they offer anything in a 215 that would work? Quite honestly the Dunlop Z1 (now officially called Z1 Star Spec) runs much wider than most normal tires, especially compared to an all season tire such as the Nero M+S. My guess is that the 205 on the Z1 would be comparable in width to a 215 on the Nero. So in this case, if these were the only two choices, I would take the Z1 with better, stickier compound that's not really that much skinnier in reality.
This suggestion of course assumes that you're looking for maximum performance, which will not be had out of an all season tire. But if you're looking for longevity, the Nero will definitely last longer (higher treadwear ratings).
Example from phillyscca.com: 225 Dunlop vs. 215 Azenis
I know 225's are wider already, but that's pretty significant. And that's against an Azenis, which also tend to run a little wider than most normal tires.
Another example from sccaforums.com
Here a pic comparing the new Direzza Z1, RE-01R, Neova, and 615. 245/40-17 size.
Some initial observations:
- The Z1 and the Neova have a 9.5" tread width, the RE-01R is approximately 9.0" wide if shaved.
And another: 235 Z1's compared with 225 Hankook RS2's
