Look for a 7-element converter. Still, a 2x may not be very good, especially on a zoom. I have an older Tamron 2x 7-element, and have never gotten a decent image from it.
The Sigma converters may be best in class (for third-party, AF teleconverters), but you must make certain that they will be compatible with your Tamron lens. If your Tamron's rear element extends to the rear of the lens, then it will not work with the Sigmas.
I suggest considering a 1.4x. One of those will give better results than a 2x.
Joe
