Double Dark Update: 2014 K1600GTL with a 205/50ZR-17 BFGoodrich g-Force COMP-2 A/S XL on the back with the .25 spacer, and an Avon Roadrider AM-26 130/70R17 back tire mounted backwards on the front, both set at 40psi.
Just returned from the running the Tail of the Dragon et. al, some 700 miles away, and the verdict is in. You can have my car tire when you pry it from my cold dead hands! We stayed in Robbinsville on Hwy 129 a couple miles South of the Dragon at an AirBnB called the "Dragon's Nest" 2 Bedrooms was all we needed, but the place had a Pantry big enough for a blow-up mattress for 2, and room enough in the living room for two more if need be. Spectacular location, for under $100 a night.
We took at least 7 passes on the Dragon, and found that it is far better after 6pm, as the traffic dies out to the point you can sometimes run the 11 mile stretch with minimum impedance... which means FAST! While there, we also ran the Cherohala Skyway on the Ocoee Loop, Six Gap in GA, Ruby Run, the Moonshiner, the Great Smoky Mountains National Park Loop, Devil's Triangle, and a bit of the Blue Ridge Parkway. I found Six Gap as challenging as the Dragon, and Devil's Triangle even more so! (Corners down to 10mph, that had to be respected, as opposed to the Dragon's 15mph corners, that can be negotiated between 30 and 50 with some practice.)
The tires performed impeccably (even when wet), and I don't even understand how. Anybody who says the contact patch is the same or bigger than motorcycle tires is just flat out wrong; but somehow the grip is near absolute anyway, even while scraping my boots and pegs (which isn't something I'd done before on the K16, but had to become comfortable with, fast!) My buddy was on a Katana 600, which should have outperformed my big tour bike, but didn't stand a chance: He couldn't corner any faster, and was hopelessly over-matched in the relatively straight sections.
As for running Double Dark on the super-slab; no contest. The car tire is a dream on the highway, so effortless to maintain a straight line, I'd run one forever even if the performance wasn't outstanding in the twisties, which it is.
Attached find a couple pictures of my first pass on the Dragon, caught by the guys at 129photos.com. This was on our way into town, so was by far my slowest pass, but you can see in the second picture how little of my rear tire was actually in contact with the road, even going slow. If you look closely at the my fairing on the first picture, you'll see a reflection of the Sheriff who issued me a warning for riding sans helmet.
Cool cop, who gave us the obligatory suggestion to pick up a whistle and tourniquet in case we left the road when no one was looking. Judging by his demeanor, and the large stack of blank warning citations on his seat, I think it's safe to say they know why we go there, and are NOT in a hurry to punish us for testing/developing our skills. As I was chatting with the Sheriff he got this weird look on his face, as if he didn't understand what he was seeing, then asked point blank: "Is that a car tire?" He rattled off some nonsense about the Dragon requiring Manufacturers' recommended tires (specifically citing slicks, which is real), but didn't pursue it further, beyond the apprehensive look covering his face.
At the end of the day, I now have 3,500 miles on the Double Dark setup and the chances of me ever going back are firmly set at ZERO. They still look fresh, and I absolutely love the way they perform!
P.s. Thanks again to all ya'll who provided guidance on the tire change, and I'm sorry I missed ya'll at the BMW MOA. Second AirBnB was about an hour North of there in Kentucky, but I couldn't see paying $85 dollars each for admission to what would have been a beer tent with live music, where I couldn't drink much beer on account of being an hour from home-base. Frankly, I wish I'd have spent another couple nights in Robbinsville.