Not current on the 320 but from memory, when you activate approach phase the box memorises the weight at that point, so if you do it too early the speeds will be in error as the weight reduces further.
1 tonne makes 1 knot difference to Vapp (which rounds up) - see Vapp computation in QRH. How many tonnes do you think an A320 burns from 80 miles before TOD? Who and what flies that accurately?! It's irrelevant and people being peeved by it need to heed Einstein's advice:
Any fool can know. The point is to understand.
It is in FCOM, I'll provide the reference later as I'm away from the iPad. It's in the characteristic speeds bulletin from the old FCOM.