WBF
Its great that you demand documentation but all you really need to do, if you do in fact fly 320s, is watch the box. Clearly your instructors never provided a reference to support your current position because they're wrong and the bulletin you linked above clearly does not suggest what you believe to be the case.
I would suggest you consider the possibility that your bulletin means what it says, that "current" weight is used for the calculation and not the weight recorded at the time of approach phase activation. I have no idea what your instructors really told you but that's dead wrong.
Here in the states we usually hold 250kts until within 8 or 10 miles of FAF so we usually activate the app phase at transition altitude. Its becoming more and more common to fly RNAV arrivals with speed constraints and allowing the phase to self activate is a nice feature that I wish more US pilots used on such approaches but they are pretty rare.