""Another problem is with the test itself, when compared to a Spitfire. Overall the accuracy of the test suffers from the fact that it was flown with a crash landed plane wirh a worn, several years old engine producing less power than usual. It was then flown against a brand new Spitfire with a 1940 engine. As shown by the test data, the turns were made in the 120mph range which is too slow for the 109 slats to be deployed, which doesn't compare the maximum turning abilities of each aircraft."
Of course the old clapped out engine theory will be presented. The data is the best available and is presented as is. I am not sure about the clapped out bit either. that is a readers assumption, and is not reflected in the report which is very thorough.
as to the slat comment ... The original author shows a fundamental lack of basic aerodynamic knowledge. Slat deployment is a function of AOA. The Slats deploy at a specific AOA every time not at an IAS. The AOA remains the same and IAS at which the slats deploy will vary as a function of G .... BUT ALWAYS AT THE SAME AOA. But the corker is the bit " the turns were made in the 120mph range which is too slow for the 109 slats to be deployed" ... you are not serious surely ! Think about it Slats are High AOA slow speed regime devices they are more likely to deploy at the slower speed (i.e. higher AOA).
For the record here are the 1G slat deployment speeds as found by the RAE in AVIA 6/2394 Messerschmitt Me.109 Handling and Manoeuvrability Tests. Its worth noting a 9Mph diff between the ASI reading and the trailing static source.
