Hi - Making a safari trip to Kenya/Tanzania; I have the Canon EF 70-200mm f/2.8 L IS USM (on a 50D) along with a 2X II Extender. I am debating if I should get the 100-400mm f/4.5-5.6 L IS USM or just use the 2.8 lens with the extender? I also have the 70-300mm f/4.5-5.6 DO IS USM which I love because of its size, but from what read 300mm may not be enough. The 100-400 is 20% less in weight, but is it worth the money if I already have the other combo? Your feedback is very much appreciated.