I got decent results from 2XTC Bower MC Pz/AF which is an 'A' TC so will give correct aperture info, and will auto focus as well.
Good for uncropped images, but cropping with it compromises IQ. As always the TC affects sharpness and contrast, but this definitely does better than the Vivitar and pentax 1.7 AFA, both of which i have tested on this lens. It is actually only marginally inferior to the 1.4 sigma TC.
I paid $50 for this in the marketplace, and all the images are post processed for sharpness and contrast.
Uncropped images of the birds that sat on my yard last wknd.
Indoor testing uncropped
Cropped
I am new to shooting birds. I will have to take the combo to the Zoo and see what it can do.