I'm now wondering how the old Perf charts were created.
Looking at the Takeoff Performance chart for a PA28 I see that the OAT region goes from -40'C to +40'C, but I seriously doubt that the PA28 was actually tested at -40'C.
Performance is determined from the Pressure (Density) Altitude.
The problem is that DA is non-linearly dependent on OAT.
Back in the 60's it was easier to pre-calculate a nomograph for the 'expected' operating range and +40'C seems like a reasonable upper limit for the USA at that time.
Now if I can determine what the DA actually is (from an expanded pre-calculated DA v OAT chart, or from a formula), then I can apply that DA to the Perf chart and on that basis the 40'C 'limit' is no longer a limit.
Unfortunately we are talking about CASA and their ineffable bureaucratic wisdom, so common-sense is not allowed.