Out of interest, I wanted to see, what the Google Advertising Cookie Opt-out Plugin does.

A quick download and glance at the source reveals:

It just sets id=OPT_OUT in the cookie for the domain doubleclick.net with an expiry date far in the future. Nothing more, nothing less.

Just like clicking opt-out on Google Ads Preferences with the added convenience that the opt-out plugin survives the deletion of all cookies.

I did confirm this for the Firefox and Chrome plugin.

So you still have to trust that Google is honoring the opt-out cookie and not track you otherwise, like through Google Analytics...

In the meantime there are other Plugins which block these tracking cookies at the client side, like Ghostery