You get reduced income, reduced manpower, increased revolt risk, and increased stability cost from wrong-culture provinces. At least some of the German cultures will probably be large enough to become "accepted" but if you continue to expand you'd probably eventually lose those too.
Oh well, thanks for the info. Formed HRE with Bohemia, but it's going rather good (had a lot of conquered lands to the east in my culture group), with me having like 3 times higher income than the #2 highest income nation (around 1700) and I regain 1 point of stability in 1 month while having an empire that encompasses half of Greece, Germany, a bit of France, Netherlands, Belgium, Denmark, African & American possessions, Poland, Latvia, Prussia/Teutonic Order, Moldavia etc. etc., so it's okay for me.