What's the EPNdB for large turbofan engines for stage 5 noise regulations?
What the FAA is calling "Stage 5" is the ICAO Chapter 14 regulations. These do no go into effect until 2017 or 2020, but that means it would be applicable for future engines such as the GE9X.

The cumulative EPNdB margin for Chapter 14 is Chapter 4 +7, or Chapter 3 +17. The exact EPNdB value depends on takeoff weight.