You definitely don't want a lens with fungus and I think you made the right call sending it back.

From what I've seen in the past 17-40 is a ways better than 16-35 I and 16-35 II is a ways better than 17-40. It stands to reason that the 16-35 I is likely a ways better than the 17-35.

Given you are using top tier optics I think you'd likely be able to tell a difference with the 17-40 and it could be marginal on performance in comparison to the others. Using anything less and think you'd be disappointed.

Dave