There is nothing in the contract that says China can't modify existing airframes the problem lies in producing unauthorized airframes. The J-11 is still based on the SU-27 airframe, it doesn't have to be an exact match but, nevertheless, it is still based on the airframe, this means China could add conards or other changes but it doesn't change the fact that they would still be producing aircraft based on the tot transfer from the SU-27 kits. Also the J-15 is said to be based off of the SU-33 prototype perchased from Ukraine, so the J-15 would be an unauthorized copy of the SU-33, even if you disagree with that statement you can't argue that China is illegaly going over the quota of aircraft based on the original Sukhoi design which includes J-11 and J-15 and no i'm not talking avionics. Let me put it this way if China sold the J-10's tot rights to another country and that country started going over the quota as well as offering the J-10 to other countries would you still have the same attitude?
i'm not supporting china on this regard but just want to clarify it china didn't go over the quota, rather, china stopped importing from sukhoi at 95 kits from the original 200 kits. at first russian didn't know what happened but soon they realized china stopped import because she now produced the kits herself. russia was of course very mad and all it could do was to stop exporting engine and this caused big pain to china. both learnd their lessions and both now moved on.