Unfortunately, this is anecdotal.
Where is your biomechanical and statisical evidence?
Even Rojo's basic NCAA statistics is superior to your reasoning.
Yes, 800m and below (<14.5/100m pace) we have seen a significant diminishing propulsion return in the lab. For slower runners heel strikers/the less efficient the performance enhancements are amplified. Conversely, unshod trained "stiffer springed" East African athletes will be less responsive to this technology.
How much faster do you think they make guys running high 3.30s (58”/400m) ?