what about boundary layer? The J-20 has a larger cross section and a longer fuselage body, i know for you it does not translate into area but for it does for air, more area exposed more radar signature, more drag.
Example F-35 and J-20 i know you will claim the J-20 is different but basicly both are the same type of aircraft seen frontally, however the J-20 is larger, this means more drag and a larger radar signature.
The F-22 has also a smaller are exposed to air and a shorter body fuselage so then times area in the equation will mean more area exposed.
Then the J-20 needs more thrust due to a larger body and heavier weight, unless it has engines like the F-119, F-135 or the new Russian T50 or I129 the J-20 won`t reach the same performance
The problem with this assumption is that you are assuming that China won't be able to come up with an engine in the same calibre by the time J-20 enters service. Going by this logic the J-20 shouldn't even exist in the first place.