There isn't a direct compare with Re-11s and Z2 yet.
But here's a tirerack.com extreme tire with the Re-11s and Dunlop Z1 star spec (previous version of Z2):
Tire Rack Tire Test - Testing the New Extremes of Extreme Performance Summer Tires - YouTube
The Bridgestone came on top but Dunlop Z1 star spec came pretty close (correct me if I am wrong, but it was a #1 tire from previous test).
The New Dunlop Z2 is suppose to be better than Dunlop Z1 star spec (according to tirerack):
Tire Test Results : Dunlop Direzza ZII Introductory Track Drive
So I would say the Dunlop Z2 should be better or even closer to the RE-11s.
But the bottom line is, I think it's safe to say "Dunlop Z2 is better than stock in rain or shine."