That's kinda what I've done, I sold my 100-300 (I became frustrated with it's af performance after using the 40-150) and was intending to get the Oly 100-400, but I could do without that bulk in my camera bag. The MC20 is therefore an option, but it isn't without compromise. At 300mm, f/8 yields better results.Reading this thread, I just thought, a MC20 on my 40-150/2.8 is a cheaper option to the 100-400, with slightly less reach.
f/5.6 is not an issue during the day, and that is similar to the 100-400 at the long end anyway.
hmm, something to think about.