Perhaps my choice of words was in poor taste but WE must have 'realistic expectations'.
China have acquired tremendous knowhow in relation to a lot of stuff from its manufacturing practices (involving countless foreign companies), and significant investment in R&D programs (2nd to USA only). Still, there are numerous fields in which 'some countries including USA' do not cooperate and/or share with China (national security considerations in large part).
China does have ample and/or intimate exposure to Russian aviation capabilities, and not surprisingly - China is not only a buyer of Russian jets but numerous Russian-origin parts are found in Chinese jets including J-20. This isn't to say that China do not know much about Western aviation capabilities in theory but intimate exposure is missing in this case.
J-20 possibly represent the very best of Chinese expertise in aviation but it is not a peer of F-22A Raptor or even a similar jet; these two are vastly different jets with distinct merits in reality. Similarly, J-20 cannot be compared to F-16 Block 70/72 standard - again lot of differences in the picture in this comparison. Do not fall for absurd comparisons on the web from some writers who are simply fulfilling their writing quotas (jobs).
Logically speaking, China have its own expertise and knowhow to share with Pakistan, and much of it is not a substitute to American input in similar domains (which is largely missing in our case regardless). I hope you get my point now.