The Saratov and Orenburg region is the only place you'll really see large numbers of germans. Its the eastern end of the black soil region (the western parts being full of russians) and its where the Germans went to farm and settle as villages. Anywhere else in Russia and there will be far more mixing, and there would be less draw since the economic operations were different from what they were used too.
That region could certainly become more German, however remember the German outflux was losing its strength by 1910 so there is not that many more years of Germans to come. You could probably double the 1900 numbers of germans to 4.5 million, but thats with immigration rates up to 25% of that which went to the US.
Of course that would also be a significant increase, it would be 4% of the Empire's population. My personal guess are that we at most would see the area along the Kazahk-Russian border with a German majority or plurality, and maybe a somewhat bigger Volga "Germany". More or less we just see one more Central Asian state, just a little farther north and populated with Germans instead of Turks or Persians.
Beside that we will just see more German enclaves along the Black Sea. They could likely be viable in the long term (rural Russian-Germans populations tended to be so), but they would be easy victims in case of a Communist revolution.
Germans tended to have have a somewhat higher growth rate than the other people of the empire*, even when we ignore immigration. Not insanely so, but enough that they will grow in size. Russian population seem to grow with 1/3 every 14-15 year. So let give the Germans a natural increase of 44% procent to 6,5 million and give a half million for immigration (and their children). So by 1914 we see 7 million Germans or around 5% of the population. This will likely have little effect on WWI. The Germans tended to be loyal to Tsar, and they will likely stay so, for the simple reason that miost Germans live in enclaves far from Germany, and the only large German territorium lies in the middle of nowhere several thousand kilometers from Germany.
*Quite logical, mostly rural population of selfowners in territorium with almost unlimited land