With the J-20, incidentally, the intended engine for the J-20 is actually supposed to have TVC, so WS-15s with TVC on a twin-engined fighter will improve roll-rate considerably.
The point about T/W is more that the J-20 is closer to a fighter-interceptor; its subsonic maneuverability is at least competitive with 4th generation designs, but its selling point is more its speed, range, and ability to supercruise.
In comparison with the Su-57, the J-20 doesn't trade off stealth for maneuverability, as with the Su-57, but rather trades off stealth for speed and range.
The latter two matter because the strategic / operational factor in an engagement is very important. Speed and range mean that if the enemy does not cluster up, you can exploit a local numerical superiority by concentrating your forces at one point of the enemy line. Even if supersonic flight means that you light up on EOTS/EODAS, you can still supercruise into the combat region before slowing down. Range also means that your bases can be dispersed throughout a massive front, and each base has massive area coverage, and when striking, you can go very deep into enemy territory.
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Besides the usual fanboy (supersonic maneuvering matters a lot in a dogfight!) vs cynic arguments, does anyone have any idea whether the J-20 can carry PL-XX/21s on its fuel pylons? It'd sacrifice stealth while doing so, but it'd provide an alternative counter-AEW&C option to the J-16 / J-11s, and what's more, the J-20s can supercruise. It might be able to obtain a Mach 1.4 supercruise capability (sufficient dry thrust to break the Mach barrier) if it has such excellent T/W, which given its high-speed design, would make it well-suited for interception (and this is a role of air superiority fighters, air superiority fighters can't strike, interceptors can't dogfight, strike fighters can't intercept).