On the A320, a 5 kts increase in Vapp, increase ldg distance roughly 8%
Throw in a little tailwind, late flare, drift, termals and late breaking as you wish.
I am not anal about a few extra knots with all of the above considered, but I am very sure a lot of those asking for a few extra knots, haven't got a clue what the effect is. On a 2500+ m rwy it is not a problem, but it might very well be on shorter rwy's.