Adding some anecdotes... there is a 1 mile long dead straight 10% grade that is the "end" of one of my weekly summer evening rides, and the fastest I have ever been able to go down it is 55mph. Even aero wheels didn't really help. However, there is a much heavier guy (50-60 pounds on me) that would do the ride with uus sometimes and he could get up to the mid 60s. A few times I tried to draft him down and couldn't even hold his wheel. Neither of us were doing any super-aero positioning... I'm sure if I dropped down into the frame and put my hands at the center of the bar I could go faster but my balls aren't big enough, especially on that road...