Ive recently switched from pepperpots to 15s on my runner, im under the impression that the 15's are better! im sure I was getting some tyrewall flex on the 13's none of that on the 15's! might be the suspension though, as I did adjust that to about half way instead of soft.
I havent found the limit of my goodyear eagle F1s yet, the car sticks to the road like glue!