I found part of my problem. You have to make the corrections while the sim itself is not running. I thought it would be like panel.cfg and those, where you can modify them, switch aircraft to another, then switch back and see the changes. But no, apparently, Weapons only sources the XML files on startup, and switching aircraft isn't enough to re-source the file.

BTW, I assume that if I want to modify the size of the 2D HUD I would modify the HudFov2d value?