I'm going to assume that you're talking about the ZX6-R you mentioned in your other thread. I don't know what a KX600re is.
First, why do you need more power? What are you trying to do with it? That bike is already faster than 99% of the vehicles on the road.
No, a pipe doesn't make much more power. It will give you a little bit more at the top end, maybe 4-5 hp, but you will lose torque in the mid-rpm range unless you re-jet the carburetors. The 01-02 ZX6-R would require a lot of modification to put an underseat exhaust on it.
If you just want to accelerate faster, change you sprockets. Go down one tooth in front and up two in the back.
"We didn't trade shoves; I did all the shoving." -Bobby Knight, 1981