Entaille, here is a link to a thread with some of the best Borla ATAK sound clip videos I've heard yet. Love the sound. Please keep in mind they are for the LS3... I've yet to find any good clips with the Borla ATAK and the L99. Hopefully they'll come out with this thread.
http://www.camaro5.com/forums/showthread.php?t=101739
EDIT: forgot to add the link