I do not understand why the position of the foil in the US BOX matters so much

I don’t think it matters once up on foil. There isn’t a logical explanation.

it feels like I CAN move my feet sufficiently to compensate

shimming is different, but mast position I think is fine.

I do think there is ergonomics issues with having the mast in the wrong place, ie touchdowns will be worse if mast is too far back, and popups are more awkward, and if a board has a narrow tail then you lose space for back foot leverage.

But purely from a forward backward trim, without straps, I haven’t personally found a reason to agree with you