It took me 2 minutes to find 2 very good threads...Some of the info isn't 100% correct but it mostly is.
Slightly stretched tires = stiffer sidewalls? - Scion FR-S Forum | Subaru BRZ Forum | Toyota 86 GT 86 Forum | AS1 Forum - FT86CLUB
Do stretched tires compromise grip/handling? - R3VLimited Forums
There is a reason Formula D cars run stretched rears. They want the most grip possible in the rear and the front to be loose. Im sure a few of you will think im bullshiting that last bit but...I cant find it now but there is a whole video on youtube from Drive talking about a 350z drifter with 700whp running 0 camber and stretched tires on the rear.