The NIV/TNIV are both dynamic equivalents. That's what you get when you go beyond a word-for-word attempt at translation.
I respect Mounce, but although the NIV (whether it's the 84 version, the TNIV, or the current one) uses dynamic equivalency more than the NASB, ESV, NRSV, NKJV, it remains in the cluster of mediating versions such as the HCSB, NAB, NET etc. There is a difference between using functional equivalence as a translation principle or as a translation philosophy. The NIV does the former. But versions that do the latter, like the GNT use dynamic equivalence as a translation philosophy. To lump the NIV in with the GNT, GWT,NCV, CEV or even The Message is foolishness. Could the HCSB be categorized as being in the latter group? Of course not. Yet the HCSB shows greater kinship to the NIV than the NLT for instance.
It's not an either/or situation. A version doesn't have to sit in either the formal camp or the dynamic equivalency camp. There is indeed a place in the middle. The NIV moves among both placements. So do the translations deemed formal at times. I have shown time and time again how dynamic many of the ESV renderings are. The ESV is not so differnt from the NIV in the grand scheme of things. Except when it comes to the issue of the ESV's awkward English --it bears a likeness to the NIV. I know that is a hard pill for many to swallow. But it's true. The ESV marketing hype --which unfortunately is also in its preface needs to be seen for what is is; h-y-p-e.
Permit me to quote from
How to Choose a Translation for all Its worth by Gordon D. Fee and Mark L. Strauss:
"So while formal equivalent translators try to proceed with a
method of formal equivalence (word-for-word replacement), their decisions are in fact determined by a
philosophy of functional equivalence (change the form whenever necessary to retain the meaning)." (p.28)