The energy going into the next harvest happens right after the blooms die. The best time to trim them back is right after the flowers die. That being said, lilacs are very resiliant. I moved into a house that had a 'tree'. We cut it back by about 1/3 taking the largest branches as close to the ground as we could. The next year, same thing. Repeated yearly until we got it under control and nice healthy branches and blooms. If your growth is covered in powdery mildew you gotta get it off because it will continue to do damage and it will spread. You might have crappy blooms this year, but at least it won't be all dead. You probably need to thin the bush anyways. If it's got PM, then there's no airflow within the bush.