I measured the lens distortion (using this method) of 90-degree versions Git2P and 7S action cameras as well as some other popular low-distortion lenses.
Firefly 7S (4.3mm lens):

a=0.0362736
b=-0.098951
c=0.0433648
GitUp Git2P (4.1mm lens):

a=-0.0016663
b=-0.0175647
c=0.0217117
Cheap $10 4.35mm lens:

a=-0.00999378
b=-0.0123347
c=0.0527412
3.87mm lens:

a=0.09937
b=-0.24381
c=0.15197
4.35mm lens:

a=0.0250548
b=-0.0622145
c=-0.00153784
Firefly 7S (4.3mm lens):
a=0.0362736
b=-0.098951
c=0.0433648
GitUp Git2P (4.1mm lens):
a=-0.0016663
b=-0.0175647
c=0.0217117
Cheap $10 4.35mm lens:
a=-0.00999378
b=-0.0123347
c=0.0527412
3.87mm lens:
a=0.09937
b=-0.24381
c=0.15197
4.35mm lens:
a=0.0250548
b=-0.0622145
c=-0.00153784
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