Tbh the prudent move here is look well past 6Mbps as
@aguerrrroooooooooooo notes
A good example is NBC sports doing 1080p @ 8Mbps, i is delivered in hds packets which allows for variable bitrate so it WILL spike some points at say 1.7MB/s.
If your line is close to it's limit with that you are going to have a bad viewing experience. As noted though most people can get 30 now with out to much hassle or cost.
You could not stream a 720p film from my servers with that speed, an untouched 720p x264 at about 6GB will run at about 900KB/s spiking to 2.5MB/s.
Public feeds are not that high quality but they will be in a couple of years, i think sky 30Mbps fibre would be the minimum now for anyone who streams and has 2-5 devices connected.