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The Tamron 2x is meant to be really good and I believe is 'non-reporting' so AF is not disabled for an f4 lens (although many tape the pins on the Canon TC to get the same effect).
The 135 f2 is an exceptional lens, forgot about that one. It is meant to very sharp at f2, but suffers a bit from vignetting when used that wide. I was considering that one before I bought the Sigma 300 2.8. I haven't seen much about the 135f2 + TC, but the 200 performs really well with the 1.4x and is quite good with the 2x. You will loose image quality with teleconverters, so you need a good lens to start with.
Terry
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