If the PLA can get the J-20 operational early with Al-31Fs, that's a big plus. Simply having 5th generation capability in the air, no matter how underpowered, is a dramatic game changer that would radically shift the regional balance of power. While the aircraft may not be sufficiently maneuverable in the air or have strong supercruise in its initial incarnations, it would still give you a stealth air superiority capability that would allow you to gain a radical BVR advantage over the 4th generation opponents that are common in the area.
The best part with early J-20s operating Al-31Fs is the ambiguity. Say, when J-20s operating 117S (it's possible) or WS-15s come up, the opponent can't tell at a glance whether you're operating with high-thrust engines or low-thrust engines. Since they're now likely to mis-estimate your capabilities, they're now liable to make mistakes that you can exploit, either overestimating your Al-31F craft or underestimating your WS-15s, leading to higher casualties.
I would call it a success if the Japanese get forced to the negotiating table because J-20s with air superiority functions show up in 2016 and they won't receive delivery of F-35s until 2018.
