1) I just tried Speedtest and it wasn't showing my typical bandwidth (about 60%). I've tried in the past several times and it usually did. So you may need to try a few times at different times of day to get a better idea.
2) A download speed of 6MBps is adequate for most Netflix shows so there's no issue there.
3) For reference you should get up to 2.75MB per second or almost 10GB per hour. When you download files especially if you use Steam you can see if you're getting roughly those results.
You won't always get full speed but Steam is usually pretty good.
4) A bill is being pushed forward to force Internet Providers to provide the bandwidth they quote. Currently they can throttle depending on usage and time of day though again you've got such a high bandwidth it shouldn't be much issue.
5) The ONLY time you might want better bandwidth is cloud-based gaming but I dislike that due to latency issues (remote server does all processing but it has to wait for your input, then processes, and finally streams the video a la Netflix so there's delay).
Microsoft does have some servers (Azure) devoted to a few XBOX ONE games but those are for offloading multiplayer calculations to reduce local CPU load. The graphics and most of the game are all processed locally on the XBOX ONE.