that means...it is better to drop below the profile couple of hundreds of feet to cater for the extra knots when starting the desceleration
No in the situation you describe: flying 320 kt selected when the managed descent profile is still based on 280 kt. It is better to forget about the whole managed descent profile or force the managed profile to be recalculated with a new managed speed of 320kt.
option 1:
-disregard the FMGS profile, it has no significance any more. You are low on the 'true' profile.
-select VS -1000 fpm or -500 fpm whatever makes ATC happy in your area
-work out your own descent and pull for OPEN DES (3 times table does magic plus a few miles for decel)
option 2:
-recalculate the profile. DL suggested to reinsert the cost-index. This does force a recalculation, same does: selecting re-direct to the TO waypoint, re-inserting QNH, temp, wind -changing or re-inserting any constraint.
However the profile is recalculated STILL assuming the MANAGED speed target of 280kt even though you are flying selected speed at this stage.
The only way to recalculate the profile properly with your 320kts is to leave the DESCENT phase temporarily to allow for a new managed speed insertion on the DES page.
You can leave the descent phase by entering the cruise phase again in case of an intermediate level-off. As described in my post above.
Option 1 is much easier. We are pilots after all.
Hope this helps