I can tell you that we have sold more sets to coupes owners than GS owners, probably because there are simply more coupes than GS cars.
The differences in suspension between a skyhook coupe and GS is very slim. The differences are a 5% change in spring rate, slightly lower factory ride height and 19" wheels
The difference is quite noticable on both, I have personally driven the GS, Coupe CC, Coupe Manual with and without our springs. I haven't driven the spyder yet.
The result will be that both a GS and a Coupe will handle the same, assuming that they had the same wheels / tires. With the stock configuration the 18 wheels will absorb more bumps and handle well, the 19's will handle slightly better and turn in will be sharper, the ride quality will not be as good.
Basically it will be a trade off between slightly improved handling against a diminished ride quality.
That was a very honest answer and to be truthful, I felt that the coupe would probably benefit more than the GS, so am still undecided. Would love to try a GS with the uprated springs, but the chances of that in the UK are slim.
I had a track focused 993 that was OK on the road,but had fantastic handling on the track, but I got to the point that if I was not 100% focused on driving it became very tiresome on normal roads.
Search a few of our customers posts and connect with them or ask me and I'll get you in touch with some of our customers. The street ride will not be significantly diminished, the difference is that our springs work with skyhook. You will be able to drive it on the street much easier than your 993, the upgrade is meant to be a proper comprimise and I think we've nailed it, but again, speak to our customers and get their opinion first hand if you're on the fence. I do believe you'll be pleased with the results.