My own understanding is that one should let the speed stay "managed' as this would be the most efficient way of getting up there
If you change the speed, you are effectively changing your cost index, so in that respect, managed speed is more efficient, however, the A320 is no rocket ship and if you are heavy it will occasionally stop climbing at all in managed speed, 500 FPM is the minimum climb rate, using one of the two methods mentioned will solve the problem.