Performance winters are FAR FAR better on a dry road than all out snow tires. It's not even close. I would urge anyone who has a sports car in a climate that sees freezing temperatures to get performance winters. Snows have far less rubber in contact with the road. That's not just for handling! Dry stopping distances will be far better in the performance winters. It's a season, not just a yearly blizzard.
Why snows? They're designed for penetrating deep snow. Slush isn't generally that deep once you put >500 lbs of car on top of it. Snow will not push away from the tire though with weight, it just compacts itself. That's where a pure snow tire is unbeatable.
Basically, I would say that a performance winter beats a snow tire in everything besides unplowed roads.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD