The real nightmare hand for 6789 on a board of AT6r seems to be 789T.

In the very best case scenario, where 6789 has two runner-runner flush draws and 789T is rainbow, I get only 2.3:1 for 6789. If both are rainbow, it's absolutely horrible at 3.3:1.

I think this is only in part due to the quality of the pair. It also comes from making the straight and then having the bigger wrap spike a J. The 5, on the other hand, is irrelevant for the lower wrap if one makes the straight.

On that board, 789T also does alright against A678 if 789T has at least one runner-runner flush draw and A678 has none. Then it's 1.9:1. If 789T has two runner-runner flush draws, it's 1.7:1 (again with A678 having none).

If 789T has two runner-runner flush draws, one of which is counterfeited by a bigger flush draw from A789, then it's 2.1:1. Same if neither has a flush draw at all.

My conclusion: Being wrapped higher is also extremely important even on a 3-gapper board where you fill the gap.