I've had the phone for several weeks now on 8.1. I've done the hard reset, with no restore, and find battery life to be abysmal. Wifi calling will connect immediately after a reboot, but i find that it will stop functioning and leave the phone app in a strange state if there is no signal. Setting it to cellular preferred helps, but wifi calling wont seemingly reconnect after it disconnects once. I don't know if its related to the APs iim attached to (cisco 1242s, new cisco 3ks, Cisco E2000 at home, at&t router/dsl modem combo at home), but this was not a problem prior to the 8.1 upgrade. Wifi calling is probably going to be broken until tmobile releases cyan. SMS messaging on wifi calling is impossible on 8.1 right now due to the carrier # information being cleared when its active. This value cannot be edited by any tools that i can find, so you're probably going to be waiting for tmobile to hopefully not EOL the phone prior to cyan. Luckily(?) being a lumia, you can roll back to wp8 if you need those features at the loss of a lot of excellent functionality.
Of course, if anyone has a working solution to any of this, please post it. Weeks of research have resulted in nothing but complaints and concerns, so a positive outcome here seems doubtful.