From 1820 on German and Austrian Immigration to the USA was
Germany -- 6,021,951
Austria --- 2,534,617
Add to this another 100,000 going to South America. Now this is not counting the children born in country.
Anyway if Germany had been able to keep those people (many left after the rev of 1848) what impact would this have had for WWI. Add another 800,000 - 1,000,000 troops x 3 from birth rate = 2.4 / 3 mm additional.
Positive - greater productivity - more troops (think from 8 armies to 12 - 14 at start of the war)
Negative - greater burden on food resources (however you would have had many more farmers to produce the crops that went unplanted for lack of workers (fertilizer is an issue -- however creative solutions are available - Night Soil anyone)