- I am not sure how channel busy is reported when channels are bonded. It is possible that reading is not representative of all channels when reporting more than a single channel together.
- Theoretical achievable data rates are based on how many channels are bonded, the capability of the client and the capability of the access points. Please see the rate charts here: http://community.arubanetworks.com/t5/Unified-Wired-Wireless-Access/TUTORIAL-ArubaOS-6-3-and-Above-802-11ac-Enable-VHT-even-if-you/m-p/142739
- Actual rates depend on distance, contention with other clients and potential interference in the space. Many of the higher rates are heavily dependent on close distances to the access points. As clients get further away from access points, their negotiated rates go down, as it becomes more difficult to send packets successfully at higher rates. Other clients that may or may not support 80mhz or even 40 mhz channels also consume part of the medium and could make an 802.11ac client de-rate if it cannot gain access to the medium in a specific amount of time. Non-802.11 Interference can also cause a client to de-rate, due to the fact that transmissions are unable to complete the first, second or third time successfully. In general, it takes more time and effort for a client to increase its rates, after de-rating that it does to de-rate.
- If you have mixed clients (AC and non-AC) and density, it is sometimes advisable to start with 20mhz channels because (1) It is the lowest common denominator for all clients (2) You have more available channels, so the likelihood of access points with overlapping channels next to each other is decreased (3) localized interference could potentially be avoided or worked around (4) 802.11ac clients will still be able to transmit at higher rates, even with 20mhz channels (4) In a mixed environment, it is statistically more likely to be able to transmit successfully at range with 20mhz channels.
*Everything mentioned above is theory geared towards deployment strategy assuming that most client deployments will be mixed and somewhat dense. It is also fairly conservative. There is nothing wrong with tuning things to get better results, because every deployment is different.