I am not convinced that a large-diameter turnaround will produce faster times than does a single cone.
The single cone does require the walker to reverse direction abruptly, but the large diameter TA requires the walker to walk more slowly around the arc, which loses time.
It's pretty much Holy Writ that the large-arc TA is superior, but I remain unconvinced.
Does anybody have any facts to help make a choice?
From a race administrator's point of view, a single cone is much preferred.
What do walkers prefer?
I suspect that the principal reason for a large-radius turn is to keep the walkers from getting tangled up with each other trying to navigate as close as they can to a single cone. It smoothes the traffic.