First, higher birth rate and net migrations are usually a (short-term) drag on GDP, not a boost.
Secondly, you can't look at Germany outside the context of being a member of the EU and Eurozone (and currently a very important creditor in the Eurozone). That has a huge impact on Germany's economy.
This is exactly right. If a few of Germany's key debtors end up defaulting and exiting the Eurozone, a prospect not so far out of the realm of possibility given the leftward political trend in those countries, then Germany could fall on hard times very quickly.
This was my thought as well. Germany has publicly declared that they will take 800k refugees (plus there are likely to be several hundred thousand undocumented immigrants). That will be a huge drag on their economy for a few years, but the long term result is projected to be very positive for their economy.
2009: $31
2013: $36.24
2014: $36.45
source:http://stats.oecd.org/Index.aspx?DataSetCode=PDB_LV
(GDP per head of population in USD ppp adjusted)
Both countries are however diverging pretty swiftly. While productivity in Japan was falling it was increasing in Germany. Moreover the recent development showed that Germany was making gains in demographics with higher birth rate and net migrations. Japan on the other hand was falling in recession several times in the last 5 years alone. Even without extrapolating the current trend, it is obvious that Japan would not do much with regard to its demographics while Germany's demographics is actually changing very quickly (and rather favorably).