In my project I move as much components (term means classes, modules, configs, etc.) as possible inside an "app-plugin" (term 'app-plugin' to avoid confusion with normal plugins). This plugin is configured as svn:external by two project-repositories.
Inside the app-plugin I also override components of other plugins, e.g. sfDoctrineGuard, because the overrided components should also not be duplicated for the two actual projects.
To achieve higher priority for the components of the app-plugin, I have to enable it before enabling all other plugins inside my project-configuration. Otherwise the normal plugins components would override the app-plugin's components.
This works very well, except when testing by using the symfony test:functional-task: The normal plugin's forms have higher priority.
My assumption (!) is that the task already initialized a sfContext but auto-loads in a different way.
A workaround is to use php test\functional\xyzActionsTest.php instead of the task.
If you have problems to find the reason I can support you by providing a small test case.