^ the confusion is with the phrase "like the J-20" -- I think you meant to say "like the J-XX". Saying J-20 conjures up images of a heavy weight stealth fighter.
If J-21 is indeed twin engined, it will already have one area better than F-35 which is redundancy in power plants. As for performance, who knows. It might have marginally larger internal payload like KF-X CAD images have shown (2 SRAAM and 4 MRAAM) from its twin engine configuration.
But if J-21 is what we're saying it is, it will not be the size of super hornet. Remember, the F-18E/F is in the weight class of F-15 and flankers.
As for the whole SAC monopolizing the naval fighter thing, I expect there to be a naval fighter competition in some stage, and by the sounds of it SAC is aiming for the medium weight with J-21. If PLANAF has a tender for naval 5th gen it'll logically be for medium weight first (J-15 will have the heavy weight category to itself for years to come), and by then SAC will be well into development with its J-21, whereas we've heard nothing from CAC about a medium weight 5th gen of their own. If the "competition" is similar to J-XX where both institutes made "proposals" rather than a fly off. If so SAC will be much ahead with a much closer to finished product.
Who knows maybe CAC can subcontract for some of J-21's components like how SAC is doing for J-20. Or maybe they can compete for a naval 5th gen (or even 6th) contract in a few decades time. But the medium weight 5th gen looks quite in SAC's favour if their plane is what it's said to be.
