Some time ago, I was told the following regarding plug/race boots:
If area of the shell around the navicular bone, roughly speaking the triangle between the heel, ankle, and ball of the toe, could be cut out entirely w/o destroying the shell, it would have no effect on skiing and/or can only make it better.
In other words, a proper boot fit should result in the foot not pressing against that area of the shell, thus leaving plenty of space for the foot to move/adjust as needed by contemporary skiing carving style.
Or, yet in other words, this whole area of the shell is there not for you, the skier, but for the structural integrity of the shell.
The consequence of the above claim is that it is almost always desirable to generously push out that area, regardless.
And, I was also told that this argument comes from Bode Miller - not making any claims to the reliability of this attribution though
Any opinions or references?
PS: For whatever it's worth, I personally happen to find that argument plausible. It had to be done for me anyway, for comfort reasons and I generally liked it. I just can't find any discussion about that ...