Siping is really great for winter traction. If you look for speedway motors out of nebraska they sell a hot knife/tire groover for the circle track guys for about $60. I got mine for adding 1/8 inch grooves to the center of mud tire lugs but you can flip the knives over in the head and use them to sipe strait across too. I highly recomend owning one because you can get the most out of a set of tires. When you get through the little bits of factory sipe and small voids a mud tire really looses alot of snow/ice and rock traction. If you can restore these with a similar groove or sipe they will work well to the last drop, sometimes even better. You can get a pretty lame set of tires to be really sticky if you do it right.
