Deviating slightly but in the same matter.
A design fatigue life is calculated on a perceived typical cycle. I have no exact figures to hand so I am generalising. The manufacturer may suggest the finite life is based upon a typical we'll say 45 min flight at say 7,000 feet. That's Ok, we know where we stand in regards to the finite life of our aircraft. Issues that may complicate the matter are for example, fitting long range fuel tanks to the wing tips of a C210. The wingspan is increased and the bending moment changes; hopefully the STC holder has examined all possible scenario's to ensure on-going airframe safety. But that particular STC is no longer worth the paper it's written on if you then add say a V.G (vortex generator) kit to said aircraft. Result differing loads on wing, not assessed by the fuel tank STC holder. Then go and add a STOL modification?, and the two previous STC's are no longer worth the paper they are written on.
The C210 was never designed to fly low level survey work, regardless of tail boom or not. Airframe loadings are significantly higher at low altitudes. Complicating the issue in the C210 (strutless) is the very low Vma at Max weight, and even much lower at a weight such as one pilot and nil baggage and minimum fuel. The C210 must be handled carefully at all design weight/speed limits.
When you add more than one (1) STC to cover an aerodynamic modification you are in effect a test pilot every time you fly the aircraft. Flutter testing may have been conducted for a fuel tank mod, but all three. No way!
Be careful folks, the aging aircraft issue is a major on-going problem.