Technically compliant - probably. Legally compliant - probably not. The whole area of compliance is a tower of babble. IoTaWatt has been tested and is compliant in the US (FCC), and Europe (CE). Those certifications are based on Espressif’s FCC and CE testing of the module itself, and subsequent emissions and immunity testing of IoTaWatt.
My opinion is that it would be impractical to consider replacing the ESP8266 in the IoTaWatt. Unlike other devices that use the ESP strictly for WiFi, IoTaWatt uses it for everything.
The ESP8266 is a very capable 80Mhz 32bit processor. I liken using it solely as a WiFi dongle to delivering mail with a Ferrari.