Yes, I am actually asking if it was comparable to F-22, F-35. etc in terms of performance.
Of course PLAAF must know how will it perform against its competitor... Otherwise why deploy a plane and put it into production if you know that your plane can't even win against your enemy? Or doesn't even have an edge.
I don't think espionage is a wise idea just for comparison of these planes. (For discussion sake) If PLAAF was to use espionage as a tool, why not just simply steal everything? and make something better out of it? rather than just knowing the specs of your competitor?
Well, technically no one really knows how one design will fare against another until they've actually engaged in mock combat. However, China doesn't need the J-20 to be as good as the F-22, and it's not about the individual performance of the planes anyways. These platforms are ultimately mission and system oriented, and they need to be good enough to fulfill their mission, not necessary to be "better" than another plane, which is arbitrary anyways.
For example, the J-20 does not need to be better than the F-22 to take out multiplier assets that the F-22 would need to sustain mission capability. If the J-20 snuck behind enemy lines and took out critical base components, AWACs, or air tankers, it would cripple the F-22's capabilities without requiring a direct confrontation. In this case, the J-20 would only have to be better than the defences it must go up against.
That said, there's a possibility that the J-20 is designed to be an F-22 killer, but I doubt the PLAAF sees the need to dedicate a plane to that task when there will only be 187 F-22s in production (though I still hold out that they might restart production when the US budget situation gets resolved and the USAF needs certain capabilities that only the F-22 or a new design could fulfill).
