Don't get too ahead of yourself now. Even if a production ready prototype of the WS-15 could be completed in 2 years, they would still need to test it with the airframe, which could take another year. In addition, 2 years would not be enough time to finish all the other testing the aircraft would have to undergo. In addition to basic airframe performance tests (some of which will probably have to be repeated when they get the new engine), the J-20 will have to undergo testing of its avionics, weapons, and subsystems, as well as stealth tests. Finally, if design revisions need to be made (and my guess is they will), they will need to retest the production prototype, and after that they will need at least a year to machine the assembly line before they're ready for production. Therefore I still think 2018-2020 is still the most reasonable estimate.