i've tested the 400DO and rented it for 2 trips
i think it's not worth the cost and trouble unless you really need f4 (which i did in Brazil, but not Tanzania). i found the IQ unsatisfactory with a 1.4x on my 40D. the IQ of the 400DO is better than the 100-400 but not by nearly as much as i'd hoped. i suspect the Canon 400 f5.6 is the IQ equal of the 400DO. the 400 f5.6 is not an unreasonable option as you will need 1/500 to 1/750 to stop animal action anyhow
if you use a long prime from a vehicle or boat where you have little control over location you must have a second body with a 100-400 or 70-200. in Tanzania i found that 80% of images came from the 5D2 and 100-400 and less than 20% from the 40D and 400DO. there is generally not time and the environment is not suited to change lenses - it's also really nice to have an assistant like the pros to hang on to the second camera
i think the most important tool for getting high quality wildlife images is a flash with better beamer for fill lighting
i seriously believe a 70-200 +1.4x is too short even on a crop-frame camera, however, if you don't really need f2.8 the f4 is a generally more useable lens with as good or better IQ