Ok, As it appears that only DFC has really done his homework here, and understands where the term "demonstrated" originated, here's the standard:
3.145 Directional stability and control
(a) There shall be no uncontrollable looping tendency in 90-degree cross winds up to a velocity equal to 0.2 Vs0 at any speed at which the aircraft may be expected to be operated upon the ground or water.
(b) All landplanes shall be demonstrated to be satisfactorily controllable with no exceptional degree of skill or alertness on the part of the pilot in power-off landings at normal landing speed and during which brakes or engine power are not used to maintain a straight path.
(c) Means shall be provided for adequate directional control during taxiing.
So the aircraft manufacturer's lawyers are going to go to court and say "our test pilots did that, and here's the report." If the test pilots did more, and you can bet that they did, the manufacturer is not going to tell you that. There are lots of things they are not going to tel you.
Cessna used to produce a document called "Getting the maximum performance from a Cessna 150". I saw it once. Try to get a copy now!
My Cessna has a stated demonstrated crosswind value, which I'm sure meets the aforementioned standard. It also has an STC'd STOL kit, which very much changes low speed handling, and stall speed. That installation does not change the demonstrated crosswind capability value though, as no revised value is provided with the kit. (No Flight Manual Supplement or placard is provided, and that's the only way to convey changed information).
Manufacturer's and writers of design standards know that some pilots have more skill than others. They must account for the lowest common denominator of pilot skill sets in the standards. Hence demonstrating that the average skill set is enough to safely control the plane.
Sometimes when I have done modification design complaince evaluation flying for Transport Canada (I'm carefully not calling it test flying, Im not going to start that debate!) I wonder to myself if I am properly simulating average skill during cross wind work. If I apply a lot of effort to seeing what the aircraft can do, I may have gone beyond a demonstration of average skill.
Pilot DAR