Sigma 100-300mm f4 w/ TC?

[greeneee]

I have a sigma 100-300mm f4 and have been doing a lot of wildlife photography (occasionally sports as well).
I am looking at getting a teleconverter for this lens, as I want to start videography. The lens has a slight focusing problem, it flickers minutely once focused. If i get the TC I will only use it for manual focus in video mode, my question is whether there is much loss of quality?

Here are some of my pictures taken with the lens:

 

[DavidW]

A 2x will always amplify any imperfections in the lens, you will see more chromatic aberrations, purple fringing and loss of sharpness. As you would need a higher ISO speed to compensate for the halving of the amount of light going into the camera, you will get more noise also.

 

[pete.rush]

1.4x TC nothing that you'll notice, a 2x TC yes you will.