The Addon Manager doesn't force the TextureMaxLoad parameter, which is in the [Display] section.
You are probably confusing it with the TEXTURE_MAX_LOAD parameter in the [Graphics] section, which IS changed bu the Addon Manager, if you selected the "Force HD textures" option, which does exactly what it says it does: forcing the maximum textures size.
The TextureMaxLoad, instead, is controlled by the slider in the Addon Manager and you don't have to "change it to match the fsx.cfg", because it's already read from there so, when you start FSX and open the Addon Manager, the value will be the one from your fsx.cfg.
This, provided you put it in the correct section. It must be in the [Display] section, if you put it manually in another section (like the [Graphics] for example), it won't have any effect on FSX, and the Addon Manager will instead use a default value that, if saved, will be put in the correct [Display] section.
The behavior you described seems to be consistent with this: maybe are looking at the TextureMaxLoad parameter in the wrong section.