I'd be very surprised if you can get G.Fast.
If you don't currently get the full 80Mb then in the majority of cases G.Fast won't be available.
In some cases it may provide a small increase in downstream but a drop on upstream is expected.
edit: to add, my comments are based on your stats, not the estimates you provided.
I would follow advice of IanBB and contact your ISP about the low sync. They should send an engineer to investigate.
320m is still the far end of G.Fast at the current specs.
Edited by j0hn83 (Wed 23-Jan-19 22:12:59)