I mostly shoot wildlife and currently own the 100-400mm and until this lens was announced, my only logical step up in terms of lens was the 300mm f4 ( besides the 150-400mm obviously, but $$$).
At first I thought it could complement the 300mm very well when I didn't need that much range but then I did the math and the price of the 50-200 + 300 is getting pretty close to the 150-400mm.
Being that birds are my main subjects, I mostly shoot at longer focal length and I would need to have the teleconverter attached to the 50-200 most of the time; which makes it hard to justify spending even more than I would for the 300mm. If I'm to spend that much money on a lens, it's because I'm looking for the best quality I can get. While the 50-200 with the teleconverter is pretty close to the 300mm I still see it as paying more for a lens that isn't as sharp ( at the focal length I shoot).
I might settle for the 40-150 as my complementary lens.
If birds are the main subject, there is simply no substitute for the 150-400. It is essentially a one-lens solution. However, it is expensive, heavy and not as flexible as the 50-200 plus a couple of TCs on your belt loop.
The 100-400 has slightly better IQ than the 50-200 +TC2.0 if you pixel-peep but the 50-200 combo shoots at 50 f/s vs 25 f/s for the 100-400. The 300F4 is better than the 100-400 unless you put a 1.4TC on. Then it is about equal but will shoot at 50 f/s.
I will say that the 50-200 with or without a TC is simply a joy in my hands. I find the smooth MF ring much better than either the clutch setup on the 300f4 or the MF ring on my 100-400.
The 50-200 with a 1.4TC is now my walk around lens, supplanting the 100-400. I have a 2.0TC on my belt loop when I really need it but I hate separating the camera from the lens in the field. Th semi-macro is simply awesome, so bugs or birds is my new mantra.
I use the 150-400 when I am not walking much. Those are my two lenses, and I don't expect to purchase another OM System lens.
