jim, I had the SZ50's on my '85, and the ride is softer than with the original Gatorbacks and wet weather performance is good, but the steering was not to my liking. Unlike the Gatorbacks, where the car seemed directly connected to the wheels, with the SZ50's there was a split-second lag before the car reacted which I didn't care for. Michelin Pilot Z4's were even worse, they drove like the car had 10 pounds of air in the tires. I was considering Bridgestone S02 or S03's, but I sold the car.
The old SZ50's and newer ones (EP, I think) have different compounds, and I'm not familiar with the new ones. The construction is the same, but I understand there are compatibility issues between the generations on some vettes with traction control. Some people '84-'85 owners like Kumhos, but I have no experience with them. Tirewise, the ride could be softened a bit more with tires less performance oriented.
Generally, I suspect the construction influences the ride more than the compound, and the trick is to get something that is both rigid enough to handle well and soft enough to be comfortable. Did I mention quiet?
While I've said the '85 isn't a lot better, it is better than the '84 and the real question is whether the ride is improved enough to justify the work involved. My only suggestion is to try and find a later model, either with or without Z51, for comparison beforehand.
The reason I say that is that part of the obnoxiousness is the short C4 wheelbase, and that would remain the same. Good luck.