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
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?