tire flex = heat.
siping tires = more flex (thats how the tread blocks splay out to provide more biting edges)
more flex = higher rolling resistance. (this is why everyone runs high tire pressure, to reduce fuel robbing flex)
from the tirerack website, they offer tire shaving services for race tires to reduce tread squirm and flex improving dry grip and tread life.
"Minimizing tread block squirm also reduces heat buildup and the risk of making the tire go "off" by overheating its tread compound."
http://www.tirerack.com/tires/tirete....jsp?techid=67
If you are ever serious about learning about the best in the tire world. best tires, best pressures, shaving, siping, wheel sizing, weights, etc. Learn from Racers. they will do anything to improve their competitive advantage.