I get the 'buffer' offered by the stat met percentage, my point is that it doesn't really matter whether it is 85% or 82% or 75%, it is only a planning figure enhancing the chance of not having to change the plan on the day.
It did matter on Atlantic trails - can you imagine the carnage of a 10 x GR4 accompanied 3 tanker trail that needed a full replan the morning of take off when one of your tankers is u/s, the 100kt head wind has appeared, and the booked airspace is time limited due to a jobs worth at ATC. Plan for a 95% stat met and then weep when its worse than that. We wept a lot. (Silent tears).