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.
Your audience might be under the assumption that the “original languages” are possessed with a kind of magic, a deep meaning that they cannot get from their plebian translations. In appealing to the original you may be reinforcing that conclusion, sowing the seeds of distrust of translation, or worse, cultivating mistaken conclusions about biblical interpretation.
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.
As a working guideline, then, I propose we evaluate translations on the basis of three criteria. A good translation (1) has a well-defined, well-reasoned, and useful translation philosophy, (2) applies that philosophy consistently over the “many parts and various ways” God has spoken to us in his word (Heb. 1:1), and (3) uses the “best of what’s around” to understand the original Hebrew and Greek text. The NLT gets an “A” in all three of these categories, as I will establish in a bit.