It’s a lot more complicated than just an even conversion rate.
Dragonflies were designed for people running 4:00 mile pace, as they are intended to be the 5000m spike for elites, Nike actually used to push the victory more for the mile and 3k and said the dragonflies were 5k-10k. They are made to work best when someone is running at that pace with good mechanics.
My theory is that the spikes help people racing right at about 4:00 mile pace the most, that’s why we aren’t seeing the 1500m/mile records drop or even be threatened, but the amount of 3:57-4:05 milers in the world has gone up by a massive amount.
In my personal experience, I’ve felt no benefit from dragonfly’s or air victory’s in racing, but they are much nicer on my tendons and calves than normal spikes and my legs thank me the next day.