One thing to remember tho....despite what people and publications say, using a teleconvertor is always gonna be a compromise.
e.g, my 300mm Tamron f/2.8 with tamrons 1.4 convertor is never gonna be quite as sharp as a Tamron 400mm f/4....
but, yes, I agree, the 300mm option plus a good quality convertor does open up a bit more flexibility
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