I’m not totally certain about your argument for the specific amount of overhead (if the receiver/sender are on mobile networks, maybe something happens to the packet headers for the first/last legs before the real internet). But doesn’t the OP already give an example where the low bit-rate codec is good: if you can compress things more then you have more of an opportunity to add forward error correction, which greatly improves the quality of calls on lossy connections. I wonder if smaller packets are less likely to be lost, and if there are important cases where multiple streams may be sent like group-calls.