Ya, it has to be a Z ball because I get different numbers with the (s) ball. I used BPB 10, 11 and 12 fairways and went both ways (back and forth) to average out the low winds, usually 2-3.
Full back spin with the (s) ball knocks 7-8 yards off every club except the 4 and 7 irons. I kept getting 10-12 on both of these irons. These were carry distances because I listen for the thump and fairways stop whatever rollout you would normally get on a green.
This was only about an hour of shooting but each club was hit a good 10 times at least. The 4 and 7 got more because I still can't figure out why they're different.
I did notice that deviation on missed hits was all over the place, a lot of times adding yardage like the 6i above. For this reason I tend to lay off the spin and choke the club. You get less deviation and more often than not, a good result. I've also learned to look at spin two ways with these clubs-1/2 spin for rollout control and full spin for turbo action on landing as well as distance control.