Re: Software&Firmware Updates released 521
The trick with WiFi calling is not so much a "special configuration", it is that it needs hardware that has the processing power and WiFi connection to deliver a quality call over WiFi. In these cases, it is largely older phones, say phones that don't have the newer audio processors or poorer WiFi connections (say, only supports 802.11g standard). The issue with those phones is not that they can't do WiFi calling, it is rather that the call clarity won't be high enough quality to meet T-Mobile's standards. Notice that even current low end phones, such as the LG L9, support WiFi calling. The other issue is that T-Mobile is adding WiFi calling at the OS level so, for example, the iPhone does not have WiFi calling, despite having the hardware capable to support it, because Apple won't let T-Mobile modify iOS. This is also why the Nexus 4, which has "pure Android" without T-Mobile modifications, does not support T-Mobile's WiFi calling.
Beyond that, as you point out, it would be no added hardware. Even if T-Mobile required Nokia to change a part to better support WiFi calling, it is unlikely changing that part would add an additional 4 mm in size. Instead, you are likely looking at replacing one small chip for another. It is also worth pointing out that there appears to be no added cost for the 521 over the 520 (despite the fact T-Mobile Prepaid is charging more, both the AT&T 520 and the MetroPCS 521 cost $99. For some reason, T-Mobile seems to be charging premium prices for all their prepaid phones, not just the 521.
It would be interesting to see a tear down of the two phones, and see what differences there are. As far as published specs, there are no differences -- same processor, same GPU, same memory, same battery, etc.