We're currently on AppVolume 2012, but are still leveraging AppStacks and haven't switched over to the newer packaging format. I started looking to change to the newer standard mainly for performance improvements, so I was curious to hear what peoples experience been like switching over.
I have done several migrations from Appstacks (2.x) to Packages/Applications (4.x). The login times were considerably reduced after the migration. Especially if a user has attached multiple appsstacks/applications. There is almost no login penalty when using Applications (4.x).