You can do it making 3 variables, a BaseSpeed, a RotationSpeed and a Boost .
At Event Begin Play you set the BaseSpeed as RotationSpeed
Create an Input Action for your boost key.
If you then click the boost key you change the RotationSpeed to your liking.
When the key is released set it back to the BaseSpeed.
Here for example I just add the Boost to the BaseSpeed
Your rotate functions then stay the same, just use the RotationSpeed to rotate.
