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I think the best is to come out with a new SSID and set with a new password.A lot of time is the autosave passwords that cause the problems. You will never know how many users are still using the old password.
In the real world seems 40MHz rock better than 80 MHz. This is true when most of users is using laptops. When more user is using mobile phone or tablet then 80 MHz will rock.Just some experience encounter.