Maybe they just didn't take the time to test it, or don't want to support it, or...
It's probably more resource-hungry than the 770 version, and takes longer to launch on the 770 maybe. There are a few blank seconds when your correspondent takes the call until he actually hears you -- due, apparently, to the 770's CPU "catching up". Apart from that everything seems to work OK (especially the secondary SIP account, yay :-)