The season they did the tests in is irrelevant. It was probably done in Hawaii or somewhere in the southern part of the US where there is no winter. Besides... as I pointed out in an earlier post, this test was done early in 1943 on the earlier 1942 F4U-1s. After which the improvements were made and the C and D models were produced. Once again, refer to THIS document for the relevant test results on the 1944, 1945 C and D models we use in the sim:
http://www.wwiiaircraftperformance.org/f4u/f4u-1d-acp.pdf
We need to stick to the facts here and not be assuming things that might or might not be a factor. These tests were conducted at different altitudes and weapon/fuel loadouts, but the planes were ALL loaded as the official documents clearly point out.
Edit: Also if you notice at the bottom of that doc you will see "Water available for approximately 8.5 minutes at combat power". Wasn't the water cooling removed with 4.11 which causes it to overheat more quickly?