We know that December sales in the UK would likely push that 8.7m towards 9m sold through. As you can see, every market saw higher annual sales in 2014 than in 2013 due to the fact that 12 months were tracked rather than just 2 months. Even markets like France saw sales increase from 126k to 290k. Tier 1 in 2013 saw 610k sales so it's likely that these numbers would have increased as well. But for now lets just calculate the baseline by adding an additional 610k which assumes that all other tier 1 countries did not see any additional sales. This takes our guess total to around 9.6m units sold through. Adding in tier 2, even if we add 20k for each region we still end up with an additional 500k+ sales. Therefore we can say that the baseline for XB1 at the end of 2014 was 8.7m cumulative and the other regions would likely take the total install base to over 10.1m worldwide at a minimum, again by end of 2014. This is a lowball estimate. It would likely have been over 10.1m.
For Q1 2015 we know that the USA sold through 0.66m, Germany sold through 0.05m and it's estimated that the UK could have sold through more than 0.1m. Therefore I would say that sell through for Q1 will be around and most likely over 1m worldwide.
Again, everything in this quote box is a guess, none of it concrete. Do not treat it as such.