All of that is irrelevant because your lens won't work with a teleconverter.
Not true if we are still talking about the Tamron 70-300 VC (newest model), at least on a Canon. I have that lens, a Canon T1i and the Tamron 1.4X C-AF1 MC4 TC and that combo works fine and AFs for me in brighter lighting.
I'm impressed that your camera works with a 1.4x, my understanding was that Canon's phase-detect AF would only work on the center point at f/8 and questionably for any higher values.
True it's more f/7.8... ish.