Here's a fairly decent tire review website: AllTyreTests -- reviews of tires, including many dozens of winter-oriented tires and comparisons to other "top" tires in the category.

No idea of their exact testing method, nor how exacting they get on comparing a given set of tires to each other (ie, on the same vehicle, the same route, same road conditions and temperatures, etc). But it's a bit of additional information about a great number of tires and how the folks at AllTyreTests think as to how those tires perform relative to other "peer" tires in the category, for whatever that might be worth to somebody shopping for tires.

An example of some of the tires reviewed for winter capabilities:
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