I'm afraid you and I disagree on how this verse is read. I do not believe it implies that the OT is sufficient; I believe it does say that the Gospel of Christ (in this case, through the oral teaching of Paul) is consistent with OT prophecy, and the OT illuminates and is the basis for the Gospel.
What scriptures? Old Testament leads you to belief that Jesus is the Christ? I don't think so. What good is the New Testament if the Old one is sufficient?
Again, and I'll leave it here, but, generally, yes, the OT can be a step to belief in Christ. It is not sufficient by itself.
But since there were pagan Greeks (multitudes) who accepted Paul's teaching without the scriptures from their infancy, it makes the scriptures irrelevant and unnecessary.
Besides, the OT prophesy is not all that clear-cut, or else the Jews in general would have recognized Christ purely on the basis of their scriptures.
One can argue that perhaps God didn't want them to, so what good is scripture is there is no God-given faith?