For what it’s worth, GSM does not boost radio power like CDMA when attached to weak cellular signal. In theory, a Republic activated phone provisioned for GSM (T-Mobile network) cellular coverage should not suffer significant battery drain as the result of keeping the cellular radio active in the presence of weak signal.
I would recommend this. Android developers have done a lot of optimization around the phone being a cellular device. I would hope that if a call starts on a poor cellular connection, the Republic Wireless app steps in and moves the call to WiFi in a timely manner.