The '85 isn't much different, and depending upon how old your tires are, you may be able to improve the ride more with newer tires. Unfortunately, the newer tires generally don't steer or track as well as the original Gatorbacks that were designed for the car.
You might try this question in the '84-'96 Forum and the Search option, as well.
right now i have firestone sz50's on it. had goodyear ultra hp's, but it seem to want to follow the grooves and hydroplane when it rains. the two lane hwy home gets alot of tractor trailer traffic and has grooves cut init that act like two little rivers running down the road when it rains. put the goodyears on and they tried to follow the grooves when it's dry and had a tendency to hydroplane when it rains. kept them about two months and tried the firestones. they dont track as bad and i love them in the rain(no more white knuckles when i get home), but they must be a harder compound, because the ride is a little worse with the firestones. its got new bilstein shocks on it. thanks guys
I have a 2000 coupe with the Z51 optioned suspension. The ride is a bit harsher that the non Z51 C5's, but it is not real bad. Our city streets are really bad and broken in places and that make the ride seem worse. When I am interstate cruising it rides like a dream..
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.