With so many variables it's not possible to get a clearcut comparison, but it was my definite impression when I used the HG612 for a few days that its performance on my line was worse than the Billion (lower DS attainable rate, more errors) and putting the Billion back reversed this deterioration. After using the Zyxel for a while I'll be trying more recent firmware and then later I'll put the Billion back.
If DLM intervenes it won't be a big issue for me - I'm not a gamer, and a small reduction in speed won't have any noticeable effect on my life.