telecon, second try

I'm looking for a 1.4x telecon for my Tamron 70-200 f2.8 that preserves the AF function and doesn't turn the pics soft.

Anyone have any experience with what works best?

I was looking at the Kenko 1.4X PRO 300 Teleconverter DGX (which is essentially a Hoya and the same one Tammy rebrands for their own use).