No penalty for roaming for calls or texts. The Defy can also roam on data services without penalty (The Moto X plans throttle roaming data after 100MB).
BTW you can't know for sure how well roaming will work until you actually have the phone. Overall satisfaction with roaming service seems to vary all over the map. You should also know these things going in: The Complete List of Caveats - Republic Wireless Wiki
One thing to consider is that you must be in a native Sprint network area to activate the DEFY XT. So you would have to take the phone to a Sprint coverage area in order to get it started up, and each time you have to factory reset it the process would have to be repeated.
SouthpawKB, customer since 2011, not a republic wireless employee
But the good news ... the following is taken from the Moto X FAQ
Moto X has been announced as the next available phone for Republic, and is considered the 'best' of the 3 phones that are coming good/better/best
Do I have to be in a native Sprint area to activate the Moto X?
No. Activation for the Moto X is on Wi-Fi.