The siping allows the tires to also remain cooler, promoting the longer tread life. The additional "edges" created provide improved traction... for both stopping and acceleration. Wet pavement is where it's really noticed.
Thats a bit of an urban myth. They do improve traction though.
However much cooling you get out of the siping is offset by the additional rolling friction from the additional traction. Tire temps end up staying the same or nearly so. You also need to rebalance and rotate siped tires more often, otherwise it causes other issues, ie excessive wheel bearing wear and uneven tire wear. Unless your in the pacific northwest or Midwest where you have to worry about black ice, snow, and constant wet conditions, there is no gain to siping tires. To do it in the south would be a waste of cash, imo.
On to the hunting!! Went out this morning with Gramps and Dad, saw a good little buck but he was on the wrong side of the fence. Will go back tonight and see if he is in at water...