I think I'm going with the Jardine GP1. I like it because it's louder than hell.
Jardine makes a lot of exhausts. I'm sure they would have one for a ZX6. A guy at work has one on his CBR600RR.
I think the stock speedo reads high anyway. I'm not sure. I never look at it anyway.
I want a faster bike but I'm not sure I want to compromise and get a more expensive, higher maintenance supersport. I still consider myself a new rider so I think this power level is about right. Perhaps next year I'll look into a used R6 or something once I get more miles under my belt.