1080p video is generally defined to be around 8-10mbit bitrate... Getting ~56mbit of video throughput across 7 clients on a 20MHz channel seems like it's not too far fetched. My office uses, let's say, a Ruckus competitor's AP's that they prematurely locked to 20MHz because "high density" and I can barely pull 80mbit from it using a 1 client : 1 AP test.
If it's possible to get more than 7 clients to stream 1080p from a single AP at 20MHz, I'd be highly impressed. But at any rate, this is one of the reasons I was saying earlier that it's probably a mistake to prematurely decide to use 20MHz channels unless it's an extremely dense setup with 12+ overlapping AP's at a given point.