Hi There,
If u haven't done so already, check out the great US kawi forum
http://www.kawiforums.com/forum/ - they have a huge sticky thread and heaps of posts on exhausts on the zx6r.
I recently went with an Arata (oz importer
http://www.doinbikes.com.au/) - great sound, but performance wise noticed little on the bum dyno. Slipons don't offer very much powerwise, so if thats ur aim, then a full system remapped with an ecu override will give the most results.
Yoshimura and Akrapovic are other popular brands amongst a stack of 'em.