Not A320 but A330/A340:
My company uses CI60 as standard but we are sometimes asked to use CI150 if winds are not favourable.
Above CI150 you are basically turning fuel into noise for very little gain in time.
On a 12hr flight if you plug in CI999 you make up 6-7minutes (well you would do if you didnt run out of gas 1000nm before destination)
If we are excessively early we'll use CI0 to save the company some ca$h but this is not mandated..... by the company.
Over SARs many companies used CI0 and just accepted they would be late..... Saved money and annoyed the very few passengers that bothered to fly!
Maverick the answer to your question lies in the huge quantities of gas we are talking about. An extra 15mins of engine time is negligable when considering the cost of Avtur.
If each aircraft in our long haul fleet safed 1tonne (1000kgs) per flight..... then we would save enough gas each day to send a A340 12hrs for free! Obviously its not really free but it is below budget and therefore falls on the good side of the balance sheet.