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.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