The authors studied SCNN1A, B, & G subunits for mutations in patients with cystic-fibrosis-like disease. In 76 patients there were 2 heterozygous for this variant. 0 out of 176 healthy controls had the variant, and 4 out of 647 controls already diagnosed with cystic fibrosis (two CFTR mutations).
The authors combine SCNN1A, B, & G variants with <2.5% minor allele frequency and find 30% of cases are carriers, while only 9% of controls. We can use this combined data to make an extremely rough estimate of penetrance: assuming an incidence of 1 in 5,000 for cystic-fibrosis-like disease (.02%), carriers of one of these variants has a .05% attributable increased risk (total risk of .07%).