well I guess you know where my vote is...
....the GSC's may look okay & be safe but the tread rubber is past it's useful life. Save them for the NCRS thing if that's a consideration down the road. You need to think about what is important for you in a tire, and how do you use your car.
Where I live the roads are a bit beat up & not glass-level so if there is water the ponding is a real hazard so I use the D3. It also seems when ever I decide to attend the Gathering I have to drive thru rain for at least 4 hrs....so I like the D3. Oh, they are noisy on cement, and asphalt too, but really noisy on cement. I don't make it a habit to drive in the rain, but if I'm out and it rains we don't cut our cruise's short. We just roll with it, most of the 5's & 6's I cruise with are on PS2's or Nitto Invo's, and some were on the OEM run-flats. I'm the only 4 in the crowd usually so I'm the only one on G/Yr's. I have no trouble keeping up and feeling in control of the wheels. Oh, I should mention that even at 30,000 miles the wet grip was tremendous, the wear bars were about 1/8" shy of the tread block's surface. That ain't too shabby in my book.
In the dry, the D3 needs heat to work. They don't have that "instant" response to input either. They slide a bit at normal operating temps, the more they're worn the more heat you need to get velcro. However, if you get them past normal temps they have a very wide band of "Hot, but not greasy"....you can smell them inside the car, they're ready. At that point the initial input is still not instant, but the now velcro like behavior makes up for it. I've never ran thru the useful Hot band... I guess I'm just chicken. I stress that be careful at normal temps as any sudden input and / or pushing of the limits of the chassis can result in that sliding feeling. Again, I've never had them break loose when I have encountered the sliding feeling....I just roll out slooooowly or keep the same input if I didn't mess up too badly. They will warn you as you can feel the limit clearly thru the butt dyno in a turn, specially a down hill off camber one....it's a little unnerving the first time it happens, but you have plenty of warning as you can feel the chassis twist in a C4 and the back feels like it's unloading. I suppose at that point if you insist upon keeping / or applying more power the back will come around on you? Again, I'm chicken so I'm not sure....but if you hold power constant and or roll out a bit she will hold the line. On a flat corner, and or a correctly cambered one the slide is very gradual and you know & feel it and it's in enough time that you can have time to think about your reaction. They are not abrupt at either end of the spectrum. All of this in the wet will blow your socks off as they are truly excellent.....but pricey. It is almost as if you are on dry pavement. I got 5 yrs & 32k miles out of my first set and I'm on my second set and I will be a buyer as long as they are made.
Oh, another negative. When they are at normal temp they pick up everything so buy mud flaps...and when they're Hot it's worse! Also, mind what G/Yr says about the operating limits for ambient temperature! These are not cold weather tires. They take about 30 mins. to get to temp at say 45*- 50*. Below 40* they take forever & don't seem to be able to hold the heat? It's not that they're dangerous. It's just if you want to push you don't have grip for that kind of driving. I think that applies to all of the summer performance tires though?
long winded, I know!
Tom