Hmm, really? I autocross every month and if this is the case, that's just a deal breaker. Did you chalk your tires and see them rollover or can you just "feel" it?
EDIT: Looking at the price difference between 215/45/18 V12 Venus and 225/45/18 RE760, I don't see why I shouldn't go for the RE760 considering the stiffer sidewall and quicker time in the dry on C&D test. Venus is ~$480 shipped with rebate, RE760 is ~$600 (215/45 is only $8 cheaper total) with rebate. But, that brings another contender in the mix: the Sumitomo HTRZ III 225/45/18 at ~$580 shipped.
Cheapest place for the V12's is onlinetires.com