I’d say check out Hankook Z212s @ edgeracing they are cheaper than RT-615s and the performance is about on the same par…
http://www.nyspeed.com/forums/showthread.php?t=26449&highlight=rt-615
http://www.nyspeed.com/forums/showthread.php?t=21304&highlight=rt-615
http://www.nyspeed.com/forums/showthread.php?t=7641&highlight=rt-615