The Su-27 per say isn't very modern. Same generation as the F-15 and the base models that the PRC has right now are pure air defense fighters. I'm not sure whether the upcoming Chinese purchase of the Su-30 variant includes TVC engines...I doubt it since the Russians haven't quite mastered them yet.
Can't say the same for the weapons systems and engines. There we have the edge and that alone in my opinion. An Su-27 or a later version like the Su-35, equipped with western engines and armaments would be tough to beat.
Look at the Korean fighter procurement going on right now. In my opinion if the Koreans could have got agreement from the Americans to allow the Russians to equip the Su-35 with westerns arms and engines, the competition would have been over in a heartbeat. But one has a rather hard time imagining the set of circumstances that would allow THAT to happen.