The historic growth numbers have often been propped up by concentrated rates of high growth (e.g. Eastern Bloc countries after the collapse of the Soviet Union).
What we can't predict is how these numbers will be affected by "the preaching work opening up" in countries where it is under ban today. If China were to lift the ban in, let's say 5 years, the growth numbers will jump up (and stay up) for a while. Putting arguements of China being a non-Christian culture aside, there's the potential of another 1 million new J-dubs over in China.