I was in Staysafe's stream earlier and he seems to think that guilds will not be able to steamroll BC the way Classic was steamrolled. It made me remember that a high end guild back in TBC (I think NIhilum??) rerolled factions and cleared all top end content IN ONE WEEK near the end of the expansion in TBC. I feel that if the talents are how they were in the last few patches of Black Temple or Sunwell that the content will get easily cleared immediately.
Which guild was it? I tried to look it up but cannot find any details about it but definitely remember reading here on MMO Champ that a top end guild did the above feat.