Extensions are irrelevant because they are not part of Gnome and are not supported by Gnome. If they were, releases would be accompanied by collections of tested, working, extensions. Their relationship to Gnome is much like that of Wordpress plugins to Wordpress. I.e., use at your own risk.
There is nothing in Gnome3 like about:config. Dconf/gconf are available, but without user documentation. ("Want to add an app to the panel? Here's how..... Want to eliminate the icons from the App Overview and see a list of names? Here's how....")
Besides, almost all extensions attempt to revive Gnome2-like capabilities that were stripped out of Gnome3. The irony of that is apparent.