I recently ran across a blog post by Dr. Mark Goodacre discussing his view that the Centurion's cry in Mark 15:39 is a sarcastic one, intending to say something like, as Dr. Goodacre put it, "Huh, truly this fellow as a son of God!"? This 'ironic' interpretation is supported positively in the article--that is to say that it doesn't seem to be Dr. Goodacre's purpose to offer criticism against the alternative (popular) view at this point.
Whatever the centurion meant, irony is present. That a centurion would utter a genuine confession that Christ is the Son of God carries a certain 'shock value' for the reader. Alternatively, that a centurion would mock Christ with sarcasm with an 'unwitting proclamation of the truth' (to use the words of Cranfield, St Mark, 460) is equally shocking.
There is little doubt that Mark loves irony. One example I enjoy is in Mark 6. After crowds are amazed at Jesus' teaching (6:2), Jesus is amazed at the unresponsiveness of crowds in his hometown (6:6). Mark weaves this theme of 'amazement' through his Gospel to create irony [See: Dwyer, "The Motif of Wonder in the Gospel of Mark," JSNT, 57 (March 1995), 45-59).]
The amazing rhetoric throughout the Gospel of Mark testifies to its unity in composition and (in my view) its veracity. In regard to rhetoric and narrative coherance, I have found Timothy J. Geddert's commentary on Mark to be helpful. It is by no means technical, but I have not found a comparable commentary that exposes the narrative coherance of Mark.
By the way, have you reflected on the content of the centurion's statement? Are you shocked? Isn't that what Mark intended? What did Mark hope his readers would leave his narrative thinking? Believing? Doing? Do you suppose Mark would be dissapointed in your response?