Is Xcode 7.x functional running macOS Sierra to submit builds to iTunes Connect?
Submitting apps built with beta versions of an OS or beta versions of Xcode are not officially supported, but you can build apps with a normal version of Xcode on a beta OS by changing the OS version prior to archiving the app inside Xcode.
You can't submit anything built with a beta OS or beta Xcode. After it becomes live is the only time Apple will accept it for the Store (not including TestFlight).
Is fully functional when you are in the process of develompent and testing in your own device locally, but is not possible to upload an archive to the itunes connect, to upload it is neccessary use the Application Loader from the XCODE 8 BETA, but only for Testflight, at this momment is not possible to upload a build to the App Store for revision...
Xcode 7 is unstable on Sierra. It will run, but crashes and hangs a lot. So much so that it's effectively unusable, which leaves you with two choices. Either upgrading your code base to Xcode 8 (Swift 3), or downgrading OS X to El Capitan.