A few years ago was a FCOM bulletin dealing with this problem of FAC speed or FMGC speed, at the end they said that it is better to believe in FMGC computed speed instead of FAC speed even if algorithm is the same, inputs are not (ZFW for FMGC and AOA for FAC).
Of course ZFW has to be correctly inserted however a 0,3 degree diff on AOA would come up with a 3 tonnes diff on FAC speed.
I have operated 320 at high gross weight a lot of time this aircraft is over powered not like 330 but I'm not worrying about.