Think about it.... if you have a tire that has stiff side walls it won't really give your car a chance to straighten up when you are taking a hard corner. Why? because the side wall flexes and takes some of that energy going sideways instead of just jerking the tail of your car side ways. I know there is other factors taken into consideration like how sticky the tire is and everything but I still think having moderate flex on the side wall isn't completely bad.
If you want to go sideways then stiff sidewalls should be ok. Catch my drift?