Its not the final last few hundredths thats the issue in the slightest (surely thats what Abbey reccomend the dyno for, as they mention how can you tell if youve just a achived a few more bhp when your mapping in-car, as you cant always feel it, a valid point ).

The pure dyno mapping issue is safe fueling and ignition for top end when on the road.

And estimating it clearly isnt the way to go, hence why cars go pop on high speed runs, as they not been mapped at that level.

You see people wanging in race fuel for safety before a high speed run on a car thats on a pump fuel map.
Well if it was on a pump fuel map that was done safely itd not need that.

But the only way is to do those high speed runs in the real world.