There's a lot of people not getting the full speed. I don't know if I can link to other forums on here? If so I can provide multiple threads.
The reason I mainly need better than FTTC is two reasons.
1. My FTTC connection is terrible. Not only do I get half the speed I should but it constantly disconnects. About 5x per day. It got a lot worse when a new housing estate got built in between the cabinet and my house. If it rains heavily, or the wind is heavy, it will disconnect and reconnect. If there's a storm it will disconnect and not come back for days. My Asus router (DSL-AC68u) wouldn't even work with the line, it is that bad. I have to use bridged HG612 just to get the internet to work. And my ping is 40ms which is terrible for gaming.
2. I mainly use the internet for watching Plex from a dedicated server I own. I actually own two, one in the Netherlands (Leaseweb) and one in Germany (Hetzner). You're supposed to "direct play" streams but my internet is too slow for movies, so I have to transcode and set a limit of 12mbps, on a 1080p movie, otherwise it buffers constantly. 4K is just out of the question. If more than one person in my household watches a 12mbps stream, it buffers. Despite syncing at ~38mbps. 300mbps will allow us to both watch at once, a direct stream of a 1080p Remux for example is around 30mbps per user. Call it 70mpbs total...leaves 230mbps overhead. Hell if we both streamed 4k...180mbps total, 120mbps overhead. Perfect!
Other than the occasional pacman update and Steam game install, I really won't see the extra 600mbps. I can always upgrade it anyway. Ping wise I've seen results from 4ms to 18ms, which is half of what I get now on my faulty connection.