Huh??? Using 'open descent' prevents the auto thrust from adding power in the descent until level off. How is more engine power that is being added to maintain the selected v/s going to help you get down faster?

Are you the guy descending at 1000 fpm with the engine power up at 55%, using full spoilers, and wondered why the ECAM is flashing Amber 'Flight Spoilers'??

And by the way, it doesn't matter what mode of the autopilot is being used. The laws of physics and aerodynamics don't change depending on what a/p mode you select.

How fast you lose altitude in your scenario will always be a function of your airspeed, configuration, and engine power.

In your scenario of needing to go down and slow down, adding engine power does not help.

When using 'open descent', the aircraft honors the speed first and then the altitude (without the addition of any engine power). If you want to get down faster, you can select a higher speed until after the altitude loss.