Section 5 Clause 1 is already US law. All of the customary units of measure are defined with respect to SI.
Section 5 Clause 2 is close to the existing law. Most items of commerce now are required to use dual measurement. Where it would change the law is mainly in the areas of road signage, weather reports, construction, and cooking shows, where the use of SI is essentially non-existent.
Section 5 Clause 3 is said by the proponent to provide an exception for road signs due to the cost and confusion. But that is hardly the only area that sudden metrification would cause problems. The gradual metrification that the US has been undergoing in daily life and commerce is slow, and while I would like to see it pick up the pace, going to pure SI overnight is not something I favor.
That said, I wonder if the use of the metric system would increase if the term
SI Swimsuit issue had a different connotation.