Our FM:
"Turbulent air penetration speeds provide an optimum margin between low and high speed buffets."
Interesting that our limitations are:
Severe Turbulence (Recommended)
Above 20,000 feet: .76 Mach/275 KIAS
Below 20,000 feet: 250 KIAS
When our company modifies Airbus FCOM wrt limitations they annotate it in the Limitations chapter (i.e. - company limitation) - in this case they did not, so I don't know where the speed discrepancy (280 vice 275) above 20,000 feet came from.