Q) Would you recommend the use of a drop-in polarizer with the Canon 400mm ƒ/2.8 for wildlife or landscape photography?
A) I would possibly use a drop-in polarizer with the 400mm if I was photographing landscapes that needed the effect. But with the 1.5 to 2 stops of light loss associated with using a polarizer, I would seldom even consider using it when photographing wildlife. The general need for speed and/or depth of field when capturing active wildlife will always mean that the loss of 2 stops is unacceptable. I carry a drop-in polarizer for my 500mm ƒ/4 telephoto, but have only used it a few times in the years I’ve been shooting in the field.
The Canon EF 500mm ƒ/4L lens with a 1.4x attached and a polarizer dropped was used to photograph a close-up section of Mount McKinley in Denali National Park, Alaska. I shot with the Canon EOS 20D at a setting of 1/250 sec. at ƒ/5.6 and an ISO of 100.