I have both cars on order, and have owned both the 996 and 997 GT3s, and tracked both.
It's amazing what the Z/28 has done. It's like Team Camaro dropped an atom bomb. But, the badassTrofeos explain the corner grip to a large extent, as they offer near Hoosier levels of stick. They really aren't in any fashion street tires and have all the drawbacks of aggressive R comps on the street. Trofeos on the GT3 and we'll see a well driven GT3 walk the Z/28.
I have no axe to grind as I'm enamored with both cars.
Even though the Z's shoes have a lot to do with the published laptimes, and while one can simply throw trofeos on the GT3 (they offer sizing close enough), the one thing the Z has going for it in this regard, is that it was engineered from the factory for this rubber. *That* is a huge benefit as those fancy dampers were tuned for trofeos.
OTOH, one thing that will suck super bad with the Z, is that the trofeos will be terrible on the street. Rain or cool days. Take your pick, you'll be hating life. I plan on a set of street tires/wheels to use around town and swap to Trofeos for track use. Or Hoosiers if I'm feeling giddy.