composer-drupal-optimizations | Saves memory and CPU cycles | Build Tool library

The extra settings only work for the root package. Consider user/site requiring 2 different packages that both specifiy different installer paths. Which one should composer use and how should it know? By ignoring those settings, unless they are specified in your root composer.json, composer circumvents any surprises/problems.

There is a way around this, you can create a post-install script that determines the correct path for you, e.g. by checking if myusername/composer_test is installed and the variable is set. This is a bit similar to how Symfony's Install-Script in the SensioDistributionBundle used to do it. It provides a fallback to determine the right directory to use based on configs and folder structure (due to changes in how the default directory structure looks at ~2.8, e.g. moving app/console to bin/console).

You could write your own install script that inspects the installed composer packages. The downside is, that just like you now have to specify the extra config in your second composer.json, you will have to specify the install scripts, plus you have to write the install script itself. So it might be a lot of extra work without gaining a lot from it.