You're not going to get any break neck take off speed by changing sprockets. In other words it's not going to launch like a 600. However if you increase the rpm speed you take off (3-4k rpms) at, you'll find the Ninja pretty quick. The 250 Ninja can beat a lot of cars and trunks from 0-60 mph.
The Kawasaki Ninja 250 does not have an electronic speedometer. It is a mechanical gear driven instrument attached to the front wheel. No electronic sensors. SpeedoDRD or Speedo healers will not work with it. Changing sprockets will not have an effect on the speedo.