According to Brian Monprode (the Michelin Rep) at Viper Days, Pilot Sports start to loose their performance after 4-5 heat cycles. The best thing is too run one seesion on new tires and let them sit over night, this will help them last longer (6-7 heat cycles). I am not saying they are junk after 6-7 cycles but they do start going away. Last season (at ****** Run Viper Days event)I had about 20+ heat cycles on a set of P.S. and put a new set on Saturday afternoon, then ran them sunday morning and the difference between the old set and the new "seasoned" set was unbeleiveable. I mean racers have always talked about the difference between new Hooziers and old ones. After that I BELEIVE, I was even lucky enough to take first place in the Modified class at that event. My first 1st place in three years, so yes new tires make a difference.
"13 heat cycles" you are just practicing, you will not be able to get really good times with tires that are that "old".