The reason I say that is, I've been running all seasons for the past 5 years. Prior to living in the high desert, I lived in Southern California for 30 years, and before that, Michigan.
In my experience, all-seasons work for their intended purpose, but there is a significant sacrifice - in EVERY season - mainly traction, whenever you actually need it.
I have run what are considered to be the top two "Ultra-High Peformance All-Season" tires out there - the Contis and the Michelins. Both exhibit this characteristic.
I'm not saying DON'T run all-seasons. I mean, I do right now. But for a performance sedan, they pretty much suck, because I have to drive at the limits of the tire - I can't drive at the limits of the car.
The car I have now (my BW goes into production this week or next) is a RWD sedan powered by an LS3 (415 HP/415 TQ) through a 6-speed auto. It is relatively mild compared to a BW.
It is disappointing enough having to drive this car at the limits of its tires... there is absolutely no way I am going to do the same thing in a Blackwing. May as well just save $100K and stick with what I have.
When my BW comes in I am going to rip off the stock run-flats and get PS4S in 275/325 for the summer. Then a set of performance winters (or perhaps all-seasons) on different wheels for when it gets cold.