Doing some research, it seems like a vBIOS limitation to fan RPM, not strictly a %.
Whatever 85% represents in terms of RPM (can be viewed in software) is the upper end the vBIOS allows - same with the lower end.
A vBIOS flash might allow the fan to spin a bit faster, but chances are there's a reason for such a limitation.
You're likely to achieve better results in cleaning out the card of dust and a reapply of the thermal paste.