Seems to me it could cause as much trouble as its worth due to ATC realising which companies fly slowly and which fly relatively fast and prioritising accordingly.
Or, my experience, which is that ATC ask the slow one in front to speed up. Low cost index on our fleets is leading to descent speeds of around 250-255kts. The majority of descents into larger airfields require us to speed up and thus negate any fuel saving from the low cost index!
PP