The Blizzaks soft rubber has the same surface traction, no matter how deep the remaining soft layer. True, if the total tread depth is less, it'll have less deep snow traction.
The new Blizzak models wear better than their original versions.
I'd buy them again if I needed the wheel package from Tirerack. I'll replace them with Nokian Hakkapelliitta LT.
For a very interesting article on winter tire design, go to
http://www.nokiantires.com/indexen.html
click on Product Line, Passenger, Winter, Winter Tire Safety.
Ken