Ok to be fair to my pictures this is a very tough photo, white sky, into the sun, at distance, white and black subject. But all I have done to these is auto white, auto tonal and resize. They are shot at 1/60, f/8 on a M 75-150, M 75-150 with TC x1.2, and DAL 50-300 on K-X body at -25 degrees C, full zoom and c. 150 yards distance. If it warms up outside I'll take better samples!
They are about as bad as can get. But you asked about loss of light, which this shows is not too big of an issue. This might give you some idea that light might not be the issue, BUT CA and other issues might come up with a TC.
PS I tend to overexpose and try and fix later in lightroom, sorry if they seem too bright.