Depth of sipes is a really important point. I'd noted that usually sipes disappear as the tires wear, leaving progressively less snow traction. But I'd never thought of comparing tires on that criterium. I HAD thought of comparing tread depth, which finalized my choice of Blizzak DMZ-3 over Michelin X-Ice.
I'd speculate that manufacturers like to make sipes shallow, since this makes the tread blocks stiffer resulting in better handling, and it makes the tires ineffective sooner. Presumably it takes better engineering to have deep sipes with firm tread blocks, and a modest manufacturer to make the sipes deep without saying anything about it.
Each year before installing my snow tires, I go to the significant trouble of using a small screwdriver to pick gravel out of the DMZ-3's sipes. Virtually every one of hundreds of sipes has gravel in it, so it has to be enough to do the sipes with gravel large enough to be seen. The idea is that many of these bits of gravel are sharp enough to slice through the rubber, and/or penetrate to the tire structure resulting in delamination due to water penetration. I can see they do damage to the tread block construction.
So, I happen to know exactly how deep the sipes are on DMZ-3's. Most of them go all the way to the bottoms of the tread blocks, and the rest of them go below the depth at which the tires are otherwise worn out. The DMZ-3's also have a second set of wear bars so you know when they get to the bottom of the special winter compound (and have a normal winter compound for the remaining tread depth).
