My original -7 rudder had accumulated too many cracks at the stiffener ends to go another year with it at Reno... it had been on my list to test out a -8 rudder for potential speed advantage and better flutter resistance and I am glad that I did it.
The -8 has a thicker skin (.20 vs .16) and folded trailing edge.
Every aircraft and loading conditions when testing are different, and my spin testing comparison between the two rudders was not meticulous... for me, the two rudders are very similar in spins.
In general cruise flight, the larger 7/9 rudder resists movement and tends to hold track better. The -8 rudder will move with very light foot pressure.
Flying at maneuvering speeds the 7/9 rudder feels like it is pushing the airplane around with heavy pressure while the -8 rudder is much more fluid and coordinated. This is quite nice.
The 7/9 rudder is much nicer tracking on the ground with the tail in the air at less than flying speeds... there is significant resistance to movement and I could drive around with the tail in the air without much attention. With the -8 rudder you have to be far more attentive.
For me, the -8 rudder is a keeper.