Of course, halving the channel width from 40 to 20 will reduce each AP's capacity in half in terms of the net throughput. Even with the slight loss of SINR with creating wider channels, in practice a Ruckus AP will still deliver great 802.11ac speeds to a fairly large area. In my apartment, I can use 80MHz channels through 2 layers of drywall and still reach 300+mbit of observed throughput to a 3-stream laptop, which is *still* above the theoretical throughput of a 20MHz client.
If you are willing to double the number of AP's to support the effort to move from 40MHz to 20MHz, you can effectively still achieve the same net throughput,which would work fine for your Netflix scenario where clients need a relatively low peak throughput (in the 5 to 10mbit range for HD streaming) but many clients need it.
Like Bill said, in practice I've found that even for denser environments, wider 5GHz channels tend to work better than one would expect, because well-behaved clients tend to try to move a finite amount of data, and allowing them to do it twice as fast (or 4 times as fast for 80MHz channels) tends to get them off the air in a proportionally shorter time.
as-per my above comment: Yes, increasing channelization will increase transmission speed. (to/from those clients that are able to support the higher channelization, and no "PHY" transmission speed change for other clients)
I'm not sure where the 6dB decrease you're talking about comes from but if you're concerned about decreasing the range of your 5Ghz connected clients, I'd expect them to be able to roam to the 2.4Ghz spectrum (which should still be at 20Mhz channelization) when they've gone beyond 5Ghz range.