Rather than speculate about the fuel savings available why not try a scientific test.
Takes a few flights to complete, but gives interesting results.
I have found that the difference between a 40CI descent (often around 260 kts) and a 300 kt descent is around 100 kg.
This has been calculated by noting the cruise fuel burn (must be stable), the descent fuel burn and the extra time spent in the cruise for a 300 kt descent as opposed to a 260kt descent. The time saving of the higher speed is less than a minute (and is at idle).
40 sectors a month gives a saving of 4 tons, which I may be wrong but thinkm equates to $3200.
Similar savings are achieved by cruising economically.
Green issues have been afforded much greater importance in recent times, so policies such as that at EZY will become the norm soon, it wil be the 340kt merchants who find themselves being vectored.
Do the controllers vectoring us towards the hold at high speed realise the size of their carbon footprint?
(By the way, I think it's down to form drag increase)