I'm assuming you want something like this.
Code:
double magnitude = Math.pow(2,(driveStick.getMagnitude())-1;
chassis.mecanumDrive_Polar(magnitude, driveStick.getDirectionDegrees(), driveStick.getRotationDirection());
this should make a pretty decent magnitude:acceleration ratio.
It is an exponential function that increases the speed of the robot when reaching the upper bounds.
You can just edit this so that it works with your drive and controls. Or, if you post the code I will do it for you.