I don't think you need to re-jet with slip-on, you might be able to adjust a screw on the carb to tune the air/fuel delivery, that's if your bike is running a bit lean. I have a full exhaust on my bike, I'm not sure if it's re-jetted but it runs pretty smooth. I haven't check the spark plugs yet though. Every bike is different, some will run rough after a slip on and some will run just fine. The fuel gauge on these ninjas are never accurate as well.