It's not about pushing coverage through B26. It's about densifying and making sure you have enough access points that UEs can reach.
The main problem with VoLTE will be existing devices not reaching cell towers. Sprint is notorious for having places where you can get a decent signal and usable DL speeds, but uploads fail (and this is not due to congestion). If you don't have a consistent uplink, VoLTE will fail even with QoS. I sure do hope VoLTE is an opt-in trial while we wait for the merger to close as the network today cannot sustain reliable VoLTE.
My dad's GS5 won't see B41 lots of places. My mom's GS7 acts like my Nexus 6 and 6P, and so does my brother's Motorola X4. The Nexus 6 and X4 seem more apt to drop B41 if not getting any data through it. Maybe the RIL is smarter. Even when I do have unusable B41, SMS and MMS do work over it.
Incidentally, I propped my phone up on my car dashboard (recording) for a minute on my drive home home earlier today in order to show somebody how bad the rain was.
The resulting video (68 MB) took over 20 minutes to upload 23 MB of over B41in Waterford, only to fly through the rest in less than a minute as soon as it jumped to B25.