A320 climb speed
 

Hello,

Very often, ATC requires us to expedite our climb, till a certain altitude/FL.

In such cases, is selection of turbulent speed (best rate) the slowest that is recommended?

Some say that passing FL 270, they changeover to MACH, and dial the speed down to 0.72M
First off, I didn't find any reference for this.
Secondly, SPEED changes over to MACH automatically at approx. FL 305; what then is the rationale behind first switching to MACH, below FL 305?

Also, the turbulent speeds are SPEED 275 knots till FL 330, and MACH 0.76 above FL 330.

Below FL 305, if asked to expedite climb, is it necessary to change to MACH first, and then select the appropriate value?

What are the optimum climb speeds to select, when asked to expedite, at any altitude/FL?

Thanks