three things:Looks like you're correct about the missing band 30 for AT&T. My nexus 6 is also missing that band and I've had no issues. Download and data performance have been identical to my AT&T phones here in Texas. Think I'm going to proceed with the preorder.
1) AT&T announced yesterday the release of "5G Evolution" in the Austin, TX area. supposed to get 400 Mbps speeds. that tech will be rolled out to 20 other cities in 2017.
2) there was a discussion on this website https://forum.xda-developers.com/galaxy-s8+/help/sm-g955f-sm-g955fd-att-lte-band-30-t3595396 about the issue on samsung phones. ShreckOpher shared his experience with this same issue.
3) hopefully you will post your experiences as otherwise, the phone, on paper, looks fantstic.
hope it works for you.