Well, as far as I know. Once you install the mod again, you should have the exact same visual you have before, no need to change the sliders all over again.
But I get what you're saying. You want you character to look good even when the mod is "off", right? Well I guess you could do this by editing first your character without the mod, and seeing what you like, and then installing the mod, and if you want to go beyond the game's max, increase the "Default" value and check in game, if the body part has gotten bigger, without moving the slider.
I'm not 100% sure this is going to work though, but it's worth a shot.
The backup thing you mentioned, it doesn't work that way. When the mod is uninstalled, the game uses it's original files to set the min default and max settings for each body part. Anything other than that would require the mod to be applied so the game look in the game's root folder (under gamecommondata/customization/) for the .xml files with the names starting with "customizationboneparamdesc". So that's your only two options I'm afraid: Unmodded game or modded game.
WHATHAFUUUK