You could avoid all the complexity issues of TOCTOU, detaching, transfer, etc by limiting the JSON feature to GitHub - tc39/proposal-immutable-arraybuffer: A TC39 proposal for immutable ArrayBuffers · GitHub or Uint8Arrays on them.
Further, if engines supported a string-decode of an Immutable ArrayBuffer as a view onto the Immutable ArrayBuffer, the performance gain you seek could even happen without any new APIs or any API changes. OTOH, engines are unlikely to provide that string-as-view optimization.