The DG part of a sigma lens is a anti-reflective coating on the rear lens element. This coating is to minimise glare from reflections of digital sensor when it is exposed. A 1.4TC (DG) would have the same reflection minimising effect but....
In my brief three year DSLR experience (20,000 exposures) and in all lighting conditions, I can honestly say glare from sensor reflection was a problem once only.
I use Nikon lenses, some old others new. None are DG equivalent, only Nikon's new 300mm f2.8VR has such coating.
Sensor reflection issues have been exaggerated by Sigma for pure marketing reasons, its is just not a big deal!
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