Get the Otus. You can always stack three 2X-III's (may need to put a extension tubes between them). Then you could cover the 55-330 mm range with some great glass. You will not be able to AF, but you were going to buy an Otus without AF anyway... so AF was not an important consideration. As for the loss of a half a dozen stops of light - You do not have to worry about that with all the DR and high ISO performance you will be getting out of that new camera of yours.
Have fun with your purchase. Be sure to post some pictures so the rest of us can drool.
Pat






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