MPEG2 isn't an audio codec, it's a suite of standards. But based on my experience writing hardware encoders for this use case, you're talking about MPEG1 Layer 2 audio.
Opus outperforms that greatly, but it's not going to outperform MPEG1 Layer 2 at 256kbps while only outputting 64kbps itself. (Will likely need >= 96kbps for that).