I wonder if we could contact salt mix mfg's and ask for precisely made samples that meet, according to their equipment, their labeled specs. Would that work?
Another thought would be seeing if a public aquarium could provide some water. Presumably, they buy in big enough batches and test consistently enough to provide consistent water.
Regardless, as a basic test if 1 person could provide enough water to run multiple tests on all the kits I think we'd have some basic info to draw some, admittedly unscientific, conclusions from.