Whenever it's coming out, the 2016 model is now looking a lot more interesting because of this tidbit from reddit:
"sherifhannaQualcomm Employee 116 points 7 days ago
One important note about Wi-Fi calling in Snapdragon 820: it's significantly enhanced vs. previous generation in Snapdragon 810 and 808.
Whereas previously only Wi-Fi signal strength was taken into account to decide whether or not to switch to/from Wi-Fi, now the IMS client software in the X12 LTE modem monitors interference levels, media metrics (e.g. voice jitter), Internet backhaul speed, as well as Internet reachability (damn captive portals!) to decide whether or not to switch the call. As you know, the Wi-Fi RSSI could be very strong but the network is slow or overloaded, so we're much smarter about transferring the call now.
AnandTech captured a quick video of our demo at the 820 launch event last november:
https://www.youtube.com/watch?v=FZRwMwcqbLM
You can see the call switching back and forth between LTE and Wi-Fi as we increased and decreased the Wi-Fi interference level.
It's significantly better than the first-gen IMS client. Enjoy

"
The handoffs between WiFi and cell have always been a little laggy on my N6 (MM and N). It's good to know that new modem tech might make that a thing of the past. As long as nobody messes with display sizes for the 6P's successor, I'm gonna want one.