We may learn over time some tidbits but, overall I suspect that the biggest revelation of Ling Wancheng and his Brother Ling Jihua not a so much a victory of US intelligence as a sign of instability in the CCP and the leadership culture.
This case is not a True coup for the FBI or CIA It's a Failure of the PRC security and shows cultural failings of the Communist party.
I would agree more or less with the above. Ling Jihua was involved in some kind of power imbroglio within CCP during the transition of power between Hu Jintao's administration and Xi Jinping's administration around 2012. He got involved by accident: his son died in a car speed accident, along with two young women in the car one dead one injured. The cover-up of the incident by the then security chief, Zhou Yongkang, who later fell from grace and was prosecuted by the current leadership, exposed the special relationship between Ling and Zhou.
Ling would not be the only or first high-ranking CPP officials leaking "secret" documents or information. Around 2012, there were also suddenly some media disclosure of financial assets of some senior Chinese leaders and their extended families, These included detailed investigation reports by New York Time and Bloomberg on former Premier Wen Jiabao and Xi Jinping. It was believed these were political maneuvers by the opponents, with Zhou Yongkang being the prime suspect.
Around the same time, the murder scandal of the wife of former Chongqing party secretary, Bo Xilai, had started to engulf a lot people, including Xi himself, which ultimately resulted in the attempted defection of Xi's security chief Wang Lijun at the US consulate in Chengdu. Wang brought with him, it was said, a lot of documents that would expose some dark secrets of the party.
I believe the documents that Ling Jihua gave to Ling Wancheng for him to take to the US are mostly of the same nature, similar to what Zhou Yongkang and Wang Lijun wanted to disclose to embarrass their opponents. They believe this would discredit their political opponents and show to the Chinese people and international community that they're merely victims of the political purge and power struggle, not the kind of villains that they were accused of.
If true, they were hardly the only sensitive political documents smuggled out of China in recent years. More than a decade ago, there were the Tiananmen Papers, disclosing the internal struggles and decision during the 1989 Tiananmen Incident. Then there were the personal recordings by the disgraced Party Secretary Zhao Ziyang, taken out to Hong Kong by someone close to Zhao.