The 15 were .1 seconds faster 0-60 than 16, and 16 .1 faster than 17, 18 same as 17 and 19 .1 slower. They claimed because the tires were progressively heavier for size and the 18's worse rubber was better than the smaller sizes was why their 0-60 same as 17.
The g loading was .1 more for each tire size from 15 to 18. Claimed this was due to wider section widths of the larger tires.
if you want your 944 to be faster get some of the 16 in forged club sport 951 wheels. Super light. But you also gotta find some sticky 16 in wheels.
When I was beating everyone in my white 951 i had the forged club sports and was running Yokohama 034 racing rain tires. I liked the way they kept grabbing thru the slides and drifts were slicks let go. The slicks had a hard or abrupt edge at the limit of grip where the rain tires were soft edged or gradually lost grip at the limit of grip. My mechanic recommended them to match my driving style over slicks. The rain tires did howl like a banshe on the highway though. But I never had to worry about driving home in the rain:
When Lindseys bought my white 951 they changed to the cup car suspension and slicks, then did not do nearly as well at the one autocross they took it to then dubbed it their street test car.