I noticed that theres a User.ini file loacted in GOG>The Witcher 2>bin>config and another located in Documents>Witcher 2>Config.

The one in the GOG folder only has Engine, Input, Mouse Input, and Rendering. The one in the documents folder has Engine, Gameplay, Graphics, Input, InputQwerty, Mouseinput, and rendering.

Which User.ini file am I supposed to edit and does it matter which one?

the one in documents is the one that editable also make sure once you make your changes to change the file to read-only and to NOT change anything in the configurator because if you do it will revert all changes back to default

In trying to optimize my EE performance, I stumbled about some tweak guides (and at first edited the wrong user.ini). So to be clear: The value "0" for entries in the user.ini means "deactivated" (like usually always), right?

I dont' see much graphical changes though, maybe I have to try more radical settings. What bugs me: Some texture upgrades in a quite near distance...