How much drafting helps
My wife, Merriann, did the Madison Open Water Swim on August 20. The exact same course as the Ironman, but 500 at the start instead of 3,000.
Significantly less drafting with the "MOWS" race. Her time for that was 1:36.02. A few weeks later at the Ironman, her time was 1:32.47. And she was not pushing towards the end, like she did in the MOWS, so you could probably take 30 seconds at least off the Ironman time.
So, about 4 minutes faster due to the effect of the mass drafting in the Ironman. I don't think she used as much drafting as she could have, as later in the course the swimmers do get spread out more. Some of the faster swimmers can probably knock another couple minutes off with good drafting technique.