I now have semi-working Sonic-over-ATT gigabit service.
Sonic won't let you order gig fiber unless the AT&T database of "what addresses they can run fiber to" is accurate. In my case it wasn't. It took several months of calling AT&T to find the right people. Eventually an AT&T sales guy offered to do an "Address validation" which means sending a tech out to my address to confirm that it's OK to put fiber there. What convinced him was that the database showed fiber at my neighbor's address but not at mine. He wouldn't do the address validation unless I ordered AT&T fiber service, so I did. In a few weeks they updated the database and came and installed fiber. AT&T fiber service came with a long 70-page online legal contract that you have to click "OK" to, or they never enable your router. Most of the provisions were objectionable. I declined to click it, and canceled the AT&T fiber service (within 14 days so I don't have to pay early termination fees). Then I ordered gigabit fiber from Sonic. Now Sonic would finally take the order!
Important: Unplug and return the existing AT&T modem. The AT&T installers who install the new service for Sonic will bring their own modem, and it has different firmware or settings than the standard AT&T one.
They got it all running after a few hours of shenanigans due to more AT&T database problems (they had routed my new service to an optical patch panel a block away that didn't exist, etc). It eventually came up and speed test showed 900+ mbit/sec, though uploads weren't completing in the speed test.
Drawbacks: YOU CAN ONLY GET ONE IP ADDRESS. Sonic apparently has no option to offer static IP addresses, not even to small business customers. I have 8 addrs on a slow 2mbit Sonic DSL line IN THE SAME HOUSE, but they can't make those work over the gigabit fiber!!!
Drawbacks: Your Internet traffic is all carried by AT&T, not by Sonic. AT&T are the bastards who do everything the NSA asks them to, up to and including diverting copies of everybody's ordinary domestic Internet traffic directly into NSA spy gear installed in AT&T buildings. I had thought that Sonic would have set up the modem to encrypt and back-haul our traffic to Sonic, so AT&T can't see any of it in plaintext. That is not what's happening in my installation. Anyone who's on this service is being wiretapped by AT&T.
Drawbacks: Sonic can't offer IPv6 addresses on such connections. Sonic has had its head screwed on backwards with regard to IPv6 forever, so this was predictable. You do get perfectly good native IPv6 service from AT&T, complete with NSA spying on every packet.
Drawbacks: In the modem's clunky menu interface, you can't turn on DHCP on WiFi without also turning it on on Ethernet. But then it tries to give useless private NAT addresses to everything on your Ethernet, even if you have other ISP uplinks and other static IP addresses and such.
Result: The speed is great, the lack of AT&T terms & conditions is great, but the service is really designed for dumb end users running Netflix. If you actually run a LAN with servers and such, there is no support from Sonic for the things that make that work (like static IP addrs). Looks like I'll be looking for static IP addrs from a different ISP that can extrude them to me over a VPN that runs over the Sonic/AT&T fiber, all because Sonic can't manage to do that itself, despite doing it on their DSL products.