I just got the 615's on my track wheels today, here's my first impressions.
The tires are not terribly loud and seem to have a very strong sidewall. The steering input is very precise with little tread squirm and a low slip angle. They are new and I haven't heat cycled them other than the one run home but their traction seems to be very good with a very linear breakaway. I won't know for sure how well they work until the next auto cross but so far I like
Oh, and the tread pattern really looks good on the car.......