6800 RPM to 9000 RPM is the sweet spot of the motor. That doesn't mean that shifting at 9k will make you go faster though. I do know from experience that done just right you can accidentally lift the wheel in 2nd gear lol.
This isn't something you read a book and learn how to do though, you learn shift points from experience.
Faster shifting will get you a lot better times then trying to get the perfect shift points. If you shift faster you lose less momentum therefore you accelerate faster, not to mention you have less downtime and have power to the ground for more time.
Again though, practice is the only way to get faster.