hopefully it isn't considered lame here to post a thread from another forum, but a lot of talk on one of the Porsche forums about this:
http://forums.pelicanparts.com/showthread.php?t=386520
That is a racing forum discussing experiences with this stuff and tire types as well. THis thread starts with a link from another thread, so there are two there to read thru. THe linked thread has a few really competitive AX guys weighing in but keep in mind you need even less thermal mass in your braking system, so they run the smallest diam tires they can get in a good width for the quickest acceleration between turns. Anyway, fair amount of info there.
I freely admit all my opinions are based on what I understand things to be and am involved in these threads to hopefully learn along with the next guy... but here you will see for a typical racer large wheels have not proven out better and other threads have pointed to needing bigger brakes (sometimes because the driver is a bad driver
) so therefore bigger wheels to cover. The late models are getting heavy, so 17" wheels start becoming the small size due to need for larger rotors at a fast track.
Admins - if it is rude to post a link elsewhere, I am cool with that and can delete this post if you want me to. I am here on your terms, not mine.....