The optimum level of an Airbus FMS take into account the Cost Index. So setting it higher, it will first increase your speed, that is for climb, cruise and descent. After reaching max speeds, and further increasing your Cost Index, it will start lowering the Optimum Level, to be even faster. It has nothing to do with fuel efficency. It is just the optimum level for your CI. To find the real Optimum Level, check the table in the AOM (It was provided in the Quick reference Handbook at my company).
And the most efficent flight has no cruise at all, it is a ballistic curve. i always try to achieve that, but usually fail to do so because of ATC and the busy skies of Europe. You can set up for the approach and make your announcement during descent also - just hand the control to the other guy in the Cockpit, he has a rating too.
Nic