The results I had when I coached at the high school level were very similar to yours (using the speed trap with touch pad on synthetic track with spikes) although I did have 4 guys who were sub 4.7 and my population at the time was almost 100% rural Caucasian.
My players, and head coach, actually fully adopted the system and became proud of the fact that their times were much closer to being the truth and soon viewed stop watch times as science fiction.