During this weeks developers' meeting our lightning talk was all about Drupal 8's ThirdPartySettingsInterface.
Here's the video introduction to this powerful new feature in Drupal.
The syntax for third party settings has changed somewhat since this talk. I've looked at the change records but am still having trouble getting my settings to save on the contact form. Would it be possible for you to put up the code for your example that uses the current syntax?
On further research, my thinking that your syntax was outdated was due to the fact that I had used this with fields and it works differently there. I posted an issue to try and get some clarification on why it is different for fields. If you have any insight, I'd appreciate a comment. https://www.drupal.org/node/2479551