Just to conclude my little test of the 8800NL R2, my conclusion is that it's a budget device with budget performance.
The VDSL2 performance is OK on my fairly short line, but the HG612 is better, and the Zyxel VMG8324-B10A (used in bridge mode) is better still.
The WiFi performance is not good at all. Using WiFi Analyzer on my phone I can see three neighbours producing stronger signals inside my house than my own 8800NL R2 did. I don't know how accurate WiFi analyzer is, but practical experiments bear out its results: with my normal router I can make a good connections right out to the farthest reaches of my garden, but that wasn't the case with the Billion.
I've now put the Billion back in its box and I'll be looking for a new home for it.