Hi, I have the, rather large, Think Tank Hydrophobia for my 400/2.8. It fits PERFECTLY when both parts of the hood are attached to the lens. The Hydrophobia clamps (Velcro) around the very front end of the hood which protrudes approximately 6ins from the front element of the lens. Therefore there is a very slim chance of rain actually reaching the lens.
I will admit that I've only had to use it on one occasion (October 2012), but that single experience allows me to wholeheartedly endorse the Hydrophobia. I used it in conditions which varied from heavy rain to driving snow. Here's an example of one of the shots I took in light snow.