POH's are written by engineers, then rewritten by the marketing department, then completely rewritten by the legal department.
At least part of the POH is the approved AFM which includes the section on limitations. i.e. approved by the FAA or equivalent.
I was involved in a mod to the landing gear of a Pitts S-1 - RV type gear (and Haigh tailwheel) instead of the bungees. We happened to find 25 kts xwing when we did our tests so submitted that in the AFM and the response was we could either leave it as 17 kts or have the authority's TP confirm it.