Canon EF 400mm f/2.8L II IS USM Lens

[Doninoz]

Does anyone know about the Canon EF 400mm f/2.8L II IS USM Lens. I want a good, long prime that is fast and works in low light. I have a chance to go on a photographers expedition to Stewart Island next February. This is just below New Zealand and hosts a lot of wildlife and icebergs. A lot of the tour we will be on a boat traveling around the Island and another 2 hour tour in a helicopter (with 2 stops on ice).

My biggest lens at the moment is the 70mm-200mm F2.8L IS L series lens. I also have the Canon Extender EF 2x II which turns it into 400mm without much loss of F-stop. If I married this to the 400mm then I have a pretty reasonable 800mm focal length. Some of the tours will be done in the dark or dusk so that is why I am looking at an F2.8.

I have looked at the Sigma 50mm-500mm as an option but I tend to think the images at the longest focal length for that lens are not as sharp as I would like.

The reason I am looking at this lens is that a friend has offered his for about half the price new. I took some photos through his 1D MKIV and was really impressed. It is a very heavy lens but the thing is, a lot of the time I would not be able to get very close to the wildlife/icebergs I want to shoot.

Is there any other option out there?

I did check out the post by Satine in 2009 on this subject...but just wanted an update or new opinions...would be appreciated.