I had no intention of being HRE until late in the game and found that by adding all my Eastern Europe provinces, then releasing them as vassals, earned me incredible IA. I did have a very large empire, but I was able to march through the entire reforms list by 1. Forcing France, Spain, and Naples to release the small number of HR provinces 2. Force release HR members from larger HR members, 3. Add all of my continental and Scandinavian territory to the HRE, 4. Release vassals (Hungary may have been worth an entire reform).