The μυστήριον for Paul is less the “secret” of Christ’s Messianic identity and more like the “surprise” that the Gospel, as it is fulfilled in the resurrection and Pentecost, goes out beyond the Jews; it goes directly to the Gentiles too, and to the ends of the earth.
The question is really a matter of “how many exegetical decisions am I going to answer in my translation.” The more you leave ambiguous, the more burden you put on the reader. The more exegetical questions you answer, the less burden you put on the reader.
The point: when translating from the Greek, these subtleties won’t always show up in translation. That’s why it probably feels “low impact.” But such questions are worth thinking about because, though subtle, the rhetorical and semantic functions are different in many contexts.