FYI, although Sonoff indicates “+20dBm output gain”, currently there’s no firmware that supports this level of amplification.
In fact, Sonoff wouldn’t be able to get the device certified because it exceeds standards for RF transmission power (for this class of device). The ceiling is around 9.6 dBm and there’s an effort underway to create a firmware version supporting a +9 dBm boost.
This device is a newcomer and there’s a lot more information about in a very informative thread by Hedda: