A quite considerable exercise in comparing the "Australian" performance figures and manufacturers AFM (by whatever name) for common light aircraft, was carried out
By whom ? and when ?
there were some startling differences.
No argument, there.
Never explained was that "Australian" performance figures were almost always more optimistic than the OEM.
Now this comment, methinks, is more than a bit over the top. I spent much of my earlier engineering life involved with these flight manuals and compared more than a few here and there. Perhaps you might cite some specific examples, with substantiating data, to support your claim ?