Within those limitations you've probably identified what is good. Gun Kote and some others really adhere best to non-stainless steel after first zinc phosphating (e.g., Parkerizing) it to provide it with a well-bonded and porous base to hang onto. Check
the instructions that Ten Ring has for applying Gun Kote (they sell it as TR Gun Kote) to get a better idea of what is involved. I have no idea whether the test samples tested for Cerakote included that level of preparation or not. It also adds another layer of work to the process, which you may find discouraging.
Cerakote certainly looks good in its own test results. I have no idea how it does for surface friction, though. They don't boast of lubricity as the moly-containing coatings do.
I'm simply unfamiliar with John Norrell's product.