Hey Joe, I've not used a Tamron 70-200 but I have used the Kenko Pro300 1.4x (mines a DGX though) with a Canon 70-200 2.8 IS MkI and MkII and I think you'll be surprized how good the IQ is with it. There's hardly any degradation with the IQ and the AF speed is still good and accurate too. Hopefully someone will have used the Tamron and can help you for definite but I'm sure you'll be happy enough with the Kenko.