I'm surprised to learn from you that it's a modem only, as its product information at the Billion website specifically states that it incorporates a 4-port Ethernet switch. Are you certain we're discussing the self same piece of kit?
I take on board what you and others have now stated, but I'm wondering if, in your own assessment of this, you've been looking for the very best speeds and nothing much else. For example, BT may well be planning to double the top FTTC speed in due course but few people will be able to take advantage of it. My guess is that only those situated 100m or less from their cabinet will benefit. For others, the increases will be minimal.
Also, much depends on one's view of the speed obtained under FTTC. For most people, I would think that they wouldn't quibble in getting anything from about 20M bps upward (since the average that people currently get on ADSL Max/ADSL2 is about 4M bps). Surely, it's only going to be gamers and the like who are going to be super-critical about the line speed?
So, just how much slower was the Billion 8200M compared to the Openreach modem?
Something else I haven't mentioned is reliability. There's increasing evidence that the Openreach modem is poorly designed, at least from the thermal point of view. More and more people are having to have them replaced, which clearly is not a good sign. Possibly, the Billion device is better in that regard? Perhaps also it'll be possible for the user to get the line stats from the Billion device, whereas we know that that's not the case with the Openreach device?