The big advantage of the RS3s was their ability to withstand continuous hot laps on the track. The Z1's had a tendency to eventually overheat in road course duty. I want to see some HPDE reviews of the ZII before I buy them. The same characteristic that made the Z1 great for Auto-X is what made it have limited usefulness on the track. Meanwhile the RS3 continues to be significantly cheaper than the Dunlops.
Rivals in my tire size are 20% more expensive than ZII's, in R-comp territory, so they aren't on my list even though they are supposed to be good tires.
RS4's are on the horizon, but I don't think I can wait long enough for them.