External GPU on MacBook Pro (Mid 2012)
Attaching an external GPU to a MacBook Pro via Thunderbolt has been tested and tried, yes. The specific GTX 1080 has not ofcourse, as it hasn't been released yet.

You will need the chassis. There's no way to connect the GPU over Thunderbolt without the chassis (or similar device - it doesn't actually have to be a metal box, but the electronics needs to be there).