Ok here goes...
Lets suppose you`re flying CI 200, you would be cruising at say M.81 on an A320 and descent speed would be around 330 kts.
If flying CI 10 (a common value these days) your descent speed would be around 250 kts.
(less noisy too, you can reduce the headsets volume, thereby saving your hears)
On the first case descent will take around, lets say, 18 minutes (wild guess, not refering to FCOM) and in the 2nd case it will take, lets say, some 27 minutes. There is a 9 minute difference from the 1st case. So that`s 9 minutes of engines operating in idle, instead of cruise thrust.
Let`s see that in detail.
An average A320 fuel consuption with engines in idle is 12 kgs per minute and in cruise 40 kgs per minute. So 9 minutes in idle means 108 kgs used. 9 minutes of cruise thrust means 360 kgs. But of course if you were flying CI 200, you`re flying faster, so instead of 9 more minutes in cruise, it will actually be around 3 or 4 minutes. But 3 minutes at cruise thrust represent 120 kgs of fuel, that`s already more than the 108 kgs of fuel for those extra 9 minutes of descent. Also CI200 will make you fly less time, but average fuel flow will be higher than with lower cost index. You also have to take that into account.
To be accurate you have to refer to FCOMs, inflight performance.
From personal experience, yes it does make a difference. It also makes a difference in fuel consumption if you do constant descent approaches. ie making an approach to London will take less fuel, thanks to their beautiful concept of CDAs
than in Milan or Paris, for example.
Safe flying
Cheers