Giving the bike more air & fuel offers more gains then adding a exhaust by it's self. Fitting the DNA filters with a fuel mod will show good results on a standard exhaust system.
If you have the cash I always recommend fitting a PCV as this is the best fuelling device by far, for a less cashed up fuel mod there are a few to chose from these days.
I know this thread is about the X&R but it has the same motor as the Z & the gains are very simular.
http://xt660.com/showthread.php?t=4509