Have the front caster and toe checked again. I had the exact same problem with my 18" OEM Michelins, and my toe was off (though not dramatically) With the heavy stress of these cars on the front tires, even slight errors in alignment can lead to premature and uneven wear, with high-performance tires. You would also be amazed how inconsistent different alignment machines are...go to a shop thats recently had an alignment "alignment"