what engines the J-20 has? i do not know.
i ask you is there any J-11B that supercuises?
J-10B that supercruises? Russia has now two aircraft that supercruise and 3 engines of 14.5 tonnes or more of thrust.
Like i said in the begining, if China has a 117 type engine yes you are right buying Su-35 does not make sense
but if they have troubles with their supercruising engine, buying a few SU-35 will make sense, but Russia wants money, they want at least 50 Su-35 for 100 117 engines.
Could the 117S speed the design of WS-15 probably yes, but who knows