Is this AE/coalition thing in empire a recent thing? Because I can remember a lot of coalitions in hre on patches 1.0-1.5 at least. Haven't really seen any problems with AE. In my 1.6-1.7 game Ottomans annexed like 5 provinces and soon after an offensive coalition war was called. Other than that I don't see members of HRE annexing huge amount of provinces in the same war. As Wiz says, they usually pace themselves.