I'm getting closer to taking the xbone plunge too. Fora does look fantastic, and nothing comparable on ps4. Drive club was/is a mess.
Have they said when the next Gran Turismo is set to hit?
Before 2017!

There's been a few stories recently about Polyphony going on a hiring spree (which some take as proof they are doubling down on finishing it), and there was an interview with Kazunori Yamauchi where he said basically that '2017 was too long to wait until'.
So... to answer your question, no, they haven't, but probably sooner rather than later.
I do hope it's worth the wait, because Microsoft has made something like 4 Forza games since Gran Turismo 6 came out.