It is a compromise to get length indeed, but I'm heavily leaning for the 400mm simply because it is longer with it being lighter and shorter as a very nice bonus/strong point.

Having 800mm on the 1D III or 1Ds III cropped to the same amount of reach puts me at the 1000mm range. That is some seriously solid reach for such small light weight package! Mount a high resolution 1.6 crop body for more insane reach if I wanted to.

Technically I could stack extenders, but that probably be relegated to landscape or static subjects if I would even bother. But I would think the IQ would be terrible at that point. V.2 handles it very well though shockingly. But I would still try it for giggles at least once. Lol.