No one can reliably hear a difference between high bitrate mp3 and lossless as long as you only listen to the track as provided. But it can matter if you transcode to a different lossy format (opus is quite a bit smaller for the same quality) or (to a lesser extent I think) if you edit the file (such as make a cut or apply compression) and then re-encode it back to mp3. Similarly, if you convert to a lower bitrate mp3 the quality won't be as good as converting to that bitrate from lossless. So only having a particular lossy format is a bit similar to DRM in that it restricts what you can do without a loss of quality.

I haven't tried it, but picard might work to tag the opus files that come with the game (requires an internet connected computer).

I don't think Falcom has ever provided a full soundtrack via game stores with an English release.