Tamron also makes a 1.4x converter that should work with the 75-300, however at the long end, you may only have AF function in very good light, or perhaps not at all. In any case you won't be shooting in poor light with this combination since at 300mm you will be at f/8 wide open.
Actually, Tamron makes two models, one seems targeted at the higher end. Sigma and Kenko also make 1.4x converters for Canon EOS system, and both are decent names. Take a gander at B&H, they carry all three brands. Look carefully at the details, they will tell which lenses will or will not work.
I have one of the tamron 1.4x units (not the high end one) but haven't tested it with decent glass yet.
Bottom line on converters, the 1.4x models usually give good results, the 2.0x models you see both good & bad reviews. I spoke with a well-known wildlife photog a couple weeks ago, he owns the Canon 2.0x converter and his opinion was "you could throw it in the river..."
I have a Canon 2.0x II converter that I picked up on the cheap from e-bay, and have tested it fairly thoroughly on my 70-200 2.8 lens. My conclusion is that it degrades image quality significantly wide open (effectively f/5.6) but stopped down 2 stops (f/11) it is as sharp as the 70-200 alone. I've concluded that the converter really is quite good, but of course it magnifies any softness or other imperfections in the lens used in front of it. It also seems to reduce contrast somewhat, though this is easily corrected in post-processing. I would really love to try it with a razor-sharp prime, say the Canon 400mm f/4 DO IS, but I don't have five grand to spare, LOL!
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