I don't think the OP should be telling one class of workers how to do the job; unless he is going to make a big big issue of it across the board... He is the Union Rep, afterall.
Afterall, this is the same office where the RESERVES are not even getting their entitlement to Saturdays off. Though, why the reserves are running beat me.
Some people just cannot help it. Paired working/P&L has just highlighted the problem even more. The managers are not going to stop people coming in early or even doing their D2Ds days before - all these counts. He who is without sin can cast the first stone
I don't like rushers or people that come in early then try to rush me or their partner. Other than that, managers know what the game is - some people are rather slow and perhaps a touch lazy, some are too fast, most are fine and get on with it.
Managers will always try to play posties off against each other. But if a postie is absolutely sure he is doing his best then it is hard to make him/her do more - even with threats.