I went out and tried the same thing. The Z4 GT3 is a lively car to begin with; it has a fair bit of lift-throttle oversteer so when you jump off the gas it really wants to rotate. Plus it has a decent amount of power, so if you're on the gas too hard it wants to rotate. It's a balancing act.
With RH tires my best lap was an 8:27.589. RS tires took that down to an 8:20.985.
In terms of feel, the car's overall balance didn't seem to change appreciably but the RS tires did feel as though they gave a bit less warning at the limit. Of course, I'm inclined to believe that RS tires
should give a bit less warning, so it's entirely possible that my perception was just placebo. Still, I definitely didn't get the feeling that the car was worse in slow corners. I had a couple of situations where the extra speed in slower corners tricked me into getting on the throttle a bit too soon, but that was my fault, not the tires' fault.
All that said, it's worth noting that in 2011 the real-life Z4 GT3's best lap in the 24 hours of Nurburgring was an
8:41.889. Now, that was a race lap, not a qualifying lap, so a really good driver on a time trial lap should be able to better that slightly, but not by as much as I did and I'm not even a really good driver so even the RH tires are a bit more grippy than they ought to be.