This may be of interest, even if irrelevant.....
The FAC accuracy for VLS in CONF FULL is about +/- 3 kt.(FCOM 1.22.40) This corresponds to about +/- 3 tonnes on Gross Weight for the A320 at MLW. I reckon most loadsheets (and thus FMGC speeds) are more accurate than that.
The only FCOM reference which is vaguely relevant to your question says the FAC computed speeds "also go to the FMGC to be used as limits for various guidance modes". I think the A/THR uses FMGC speeds but respects VLS in CONF FULL .. but I could be wrong!
Cheers, TP