just an update on this...
after running the Z3 tires all of last year I noticed that around 4/32nds the tread compound changed drastically. The tires did not grip nearly as well, would heat up faster, but they would wear much more slowly. The first 2 weekends they wore down to the 4/32nds mark, and the next 3 they wore down to 3/32nds. I was not too happy with the performance and fast initial wear, so I spoke with a dunlop rep a friend introduced me to. He mentioned the Z3 is targeted specifically for autocross and if you exceed 140 degrees across the tire, they wear very quickly and lose a lot of their initial grip. I expect other recent, more focused, autox tires will act the same.
I went to a yoko a052 for this year. It starts at 6/32nds instead of 8/32nds so it's not autox legal as a street tire, but it does have a 200tw rating, so for some race classes it still qualifies as a street tire. Supposedly these tires have a higher temp range they like to be in, a lot closer to a slick than a street tire, so we'll see how they get on.