Some time ago Sigma introduced DG lens series that differ mostly in improved coating that minimizes sensor to lens elements reflexions. Many lens models received that treatment including their two teleconverters: 1.4x and 2.0x. How that different coating influences pic quality and is it really an improvement, I have explored simply by shooting the same subject while switching teleconverters. Here only 1.4x TC was used, both DG and older non-DG versions. Two very different Sigma lenses were used, along with Nikon D200: 150mm f/2.8 APO Macro DG and 50-500mm f/4-6.3 APO RF. All the camera settings (like aperture, exposure, white balance) were equal.
Pics are equaly processed, the same picture crop extracted and assembled side by side. Here just two comparisons are presented but all the other made lead to same conclusions:
DG version betters in contrast, but fails in colour fidelity. It has pinkish colour cast, while non-DG converter gives more neutral colour. Resolution seems to be equal. SO if you are into buying Sigma teleconverter and can get (cheaper) old non-DG version, go for it. Despite all the efforts, I couldn't provoke situation where DG version would show any significant advantage over older non-DG version.