I can't remember hearing of anyone around here who have had their street tire compound harden up to the point of being noticed. But I'm sure someone will chime in if they've had this,
There was a little discussion on tires in an earlier post in this forum.
http://www.seccs.org/forums/viewtopic.php?t=832
It sounds like you want to stay on your stock wheels or keep the same wheel size, is that right?