I've just heard that DJI has accepted my petition's request and will be adding an adjustable speed Cine mode via firmware. 

They haven't given an exact date and they haven't said if it will be across all drones using the fly app but I will of course insist that it's important it works across every drone!

Once I get more details I will update them here. 

Thanks to everyone who signed this petition, couldn't have done it without you! :)