Omanson, R. L., & Metzger, B. M. (2006). A Textual Guide to the Greek New Testament: an adaptation of Bruce M. Metzger’s Textual commentary for the needs of translators (pp. 308–310). Stuttgart: Deutsche Bibelgesellschaft.

"Since the earliest manuscripts of the NT are without systematic punctuation, editors and translators must insert such marks of punctuation as seems to be appropriate to the syntax and meaning. The present passage has been the object of much discussion as to whether or not Paul intended to refer to ὁ Χριστὸς (the Messiah) as θεός (God). Sanday and Headlam (The Epistle to the Romans, p. 233) wrote that scholars had probably discussed the interpretation of this passage more than any other verse in the NT. Fitzmyer’s bibliographic note (Romans, pp. 556–57) lists 24 articles in scholarly journals on the punctuation and interpretation of this verse. The chief interpretations are the following:
(a) Placing a comma after σάρκα (flesh) and referring the following words to ὁ Χριστὸς, that is, “Christ according to the flesh, who is God over all, blessed for ever.” This is the punctuation in the UBS4 text (followed by NIV).
(b) Placing a point (either a colon or a full stop) after σάρκα and taking the following words as a clause independent of ὁ Χριστὸς (UBS1,2,3). Several translations (see RSV, NEB, REB, TEV, FC footnote) ...With this punctuation, “Christ is thus the climax of the prerogatives of the Israelites as Messiah, and Paul praises God for it” (Fitzmyer, Romans, p. 548).
(c) Placing a comma after σάρκα and a point (a colon or a period) after πάντων. This, which is a modification of (b), is to be translated “Christ according to the flesh, who is over all. God be [or, is] blessed for ever!” (see NEB footnote). With this punctuation, Paul would “acknowledge the natural descent of Christ and assert his preeminence among Israel’s prerogatives, and then praise God in a doxology because of it” (Fitzmyer, p. 548).
The evidence from the Church Fathers, who almost all agreed in understanding the passage as referring to ὁ Χριστὸς, is of relatively minor significance since this tradition of interpretation originated at a time after Paul wrote (that is, dictated; see 16:22) the passage. The opposing fact that four uncial manuscripts and at least twenty-six minuscules have a point after σάρκα, either by the original copyist or by later correctors, likewise is of little significance for the same reason.
The punctuation in the UBS4 text is preferred for the following reasons:

(1) The interpretation that refers the passage to Christ suits the structure of the sentence, whereas the interpretation that takes the words as a grammatically unconnected doxology to God the Father is awkward and unnatural.
(2) If the clause ὁ ὥν κ.τ.λ. is a doxology to God the Father, the word ὥν is unnecessary, for “he who is God over all” is most simply represented by ὁ ἐπὶ πάντων θεός. The presence of the participle ὥν suggests that the clause functions as a relative clause (not “he who is,” but “who is”), and thus describes ὁ Χριστὸς as being “God over all.”
(3) Doxologies in Paul’s letters are never unconnected grammatically, but always attach themselves to that which precedes: with ὁ, ἐστιν (Rom 1:25); with ὁ ὥν (2 Cor 11:31); with ᾧ (Gal 1:5; 2 Tim 4:18; cf. Heb 13:21; 1 Pet 4:11); with αὐτῷ (Rom 11:36; Eph 3:21; cf. 1 Pet 5:11; 2 Pet 3:18); with τῷ δὲ θεῷ (Phil 4:20; 1 Tim 1:17).
(4) Doxologies which are grammatically unconnected, not only in the Bible but also in Semitic inscriptions, are constructed differently; the verb or verbal adjective “blessed” always precedes the name of God, and never follows it, as here.
(5) In the light of the context, in which Paul speaks of his sorrow over Israel’s unbelief, there seems to be no psychological explanation to account for the introduction of a doxology to God the Father at this point.
On the other hand, Paul nowhere else in his genuine epistles (that is, the letters which scholars agree were written by Paul; Titus 2:13 is generally regarded as not written by Paul) ever designates ὁ Χριστὸς as θεός. On the basis of the general perspective of Paul’s theology, it is difficult to think that Paul would have expressed Christ’s greatness by calling him God blessed for ever. So punctuation (b) or (c) is also possible.
It is also possible that a copyist accidentally wrote ὁ ὥν (the one being) for ὧν ὁ (of whom). That is, the original text may have read “4 … of whom the sonship (ὧν ἡ υἱοθεσία), the glory … 5 of whom the patriarchs (ὧν οἱ πατέρες), and from whom (ἐξ ὧν) the Messiah according to the flesh, of whom God over all (ὧν ὁ ἐπὶ πάντων θεός).…” This punctuation “would make Paul assert that Christ by natural descent is a prerogative of Israel, but that God is the one who is preeminent and blest forever, in effect, Israel’s ninth prerogative” (Fitzmyer, p. 549). It should be noted, however, that there is no evidence in the manuscripts to support such a correction of the text.

Omanson, R. L., & Metzger, B. M. (2006). A Textual Guide to the Greek New Testament: an adaptation of Bruce M. Metzger’s Textual commentary for the needs of translators (pp. 308–310). Stuttgart: Deutsche Bibelgesellschaft."