Other than possibly gaining performance from Go vs PHP, as well as less memory consumption. I have no idea how large scale Next/Own-cloud implementations behave

Other than possibly gaining performance from Go vs PHP, as well as less memory consumption. I have no idea how large scale Next/Own-cloud implementations behave, but i'd assume you just throw more hardware after it. What will change though is the relatively well tested and hardened code that is Owncloud today. Switching to Go doesn't just magically make your webapp secure.