That’s not how these numbers fit together. Using rekognition, you would create a collection of faces based off your image corpus and then ask the service if a provided image has a match against anything in your collection. Lowering your confidence threshold, invariant of your sample size, is by definition going to increase your false positive rate while lowering your false negative rate.

I think it would be worthwhile to compare a mixed test set and see false positives/negatives at various confidence thresholds, because the fear (IMO) is that law enforcement will minimize false negatives at the expense of a huge increase in false positives.