All very interesting. It is clear, from my experience, that different air rifles respond differently. It was very apparent on my 1970s Airsporter and, now, on my otherwise excellent Omega, that the effects are marked and, moreover, there seems to be a tipping point, at which point the power falls away significantly, causing the sights to have to be raised by nearly 2 inches at 25-30yds.
I have not done sufficient tests to find out what happens on other air rifles. I had rather considered the Airsporter to be a 'one-off' but this is not the case. I think the Omega has a synthetic seal whereas the Airsporter uses a leather seal.
In some export markets, the impact of hot weather must be significant.
I wonder what tests the manufacturers have carried out. It seems to me there is a big hole in the knowledge/research over this topic. It would be helpful if research pointed in a particular direction to say that factor 'x' was the biggest cause of the problem, for example, but we do not know this and can only speculate, without 100% certainty that a particular cause is for the most part responsible.