So, in line with ejs's suggestion, I also figured the only way to find it out was to try it...so I bought a Draytek 2760 modem/router! The results were interesting...
Upstream sync did improve, back to the level I used to get with a HH5a - around 620k versus the 520k I get with my Billion router, so about a 20% improvement. So it looks like it is a chipset thing.
However, the downstream performance was dreadful! The Draytek could only manage just under 11400k versus the 14999k I'm getting with the Billion - so the Billion is over 30% better than the Draytek. I'm pretty sure that I'm currently banded at 15M so the Billion can probably do better if DLM relents. Again, the HH5a used to give the same downstream sync as the Draytek is now doing, so also looks like a chipset thing.
As you can imagine, the Draytek is being returned! Interestingly, the Draytek also report 33dB downstream attenuation versus the 27.5dB reported by the Billion, and the Draytek also showed a max data rate of only 13500k versus the 19500k I see from the Billion (and the 14999k actual sync I'm getting with the Billion...)
Conclusion? If you're on a long line, don't worry about whether your cabinet is ECI or Huawei...just go with Broadcom chipsets.