I had the opportunity to ride a DarkSide bike, it belonged to one of the riders in my class. He liked to ride coast to coast non stop eating Skittles, and drinking RedBull all the way.
In a straight line his bike felt solid. Running the weave at 14 feet was a challenge. I felt myself having to wrestle the bike up off the flat bottom up onto the corner, or edge to go around a cone, then back down onto the flat bottom, then up onto the edge on the other side to go around the next cone. I was unable to do it when the cones were 12 feet apart.
In the circle I could not achieve full lock with a hard inside lean. The bike kept wanting to tip back outside. During the times I did get it up on the edge, I had a teetering sensation. I was unable to do the offset as the quick back and forth quick handlebar transition needed to negotiate the exercise came too slowly. I could not flick the bike back and forth, side to side. The full lock U-Turn from a standing stop went wide and upright.
He and I talked about it for a while, and for what he wanted to do, the car tire served him well. He wanted to buy one tire, make it last for several seasons, and run super slab all the time. With the amount of miles he racks up, he didn't want to be bothered with having to change tires several times in the riding season.
Now that's my experience, and it was very limited. For anybody who rides DarkSide and swears by it, go for it. Enjoy. I was curious to know that since you like running a car tire on the rear, have you ever considered running one on the front too? If not, why? You would get the same extra long life out of the front that you do with the rear, with no fear of cupping whatsoever.