The KW41Z family of MCUs support the SMAC communication protocol and have considerable transmission power (configurable between -30 dBm to 3.5 dBm).
Tx power can change depending on external factors, such as physical obstructions, network congestion, and possible electronic interference on the same range of frequencies that you're working in. You could always improve your Tx power by adding an external PA, could you please also take a look at this Reference Design implementing it?
You can read more about this device in the KW41Z's Product Summary page, here.
