M 0.86 will give you a greater protection against LOW speed buffet, but degrade your margins to HIGH speed buffet.
Boeing's leter versions of the B777 provide both a low speed and a high speed awareness yellow 'bar' on the ASI (Not retro-fitted to earlier models). the lower bar provides 1.3G awareness of LOW speed buffet, the upper bar provides 1.3G awareness of HIGH speed buffet.
Fly the 'happy medium', i.e. the recommended Turbulence Penetration Speed
Regards,
Old Smokey