teleconverters...

[LL III]

I'm toying with the idea of purchasing a teleconveter for my Tamron 70-200 f2.8. I would greatly appreciate thoughts, suggestions, recommendations.

[inkista]

The bigger the magnification, the slower your lens gets and the softer things get. A 1.4x adds a stop to the max. aperture a 2x adds two stops (i.e.,

1.4x+70-200 f/2.8 => 98-280 f/4
2x + 70-200 f/2.8 => 140-400 f/5.6

You may also get some AF-speed hit as well, since there's less light for the camera's AF system to "see" by. The image quality will not be as good as actually having the longer lens, but it could be good enough.

[sk66]

I was surprised at how well my Nikon 70-200 VRII performs with the Nikon 2xTCIII....

But that's a little different than the Tamron with a lesser TC. Your results will likely not be quite as good with 2x... I would suggest a Kenko TelePro Plus 1.4x.
I have one and it works quite well.