Douglas Atchison Campbell, a strong proponent of the subjective genitive view has published an article in the latest issue of JBL in which he argues that 2 Corinthians 4:13 provides evidence for Jesus being the subject of the verb. He concludes:
this text provides an instance of the verb πιστεύω being used in Paul with Christ as its implicit subject, with a further application of his ongoing belief to the Christian by way of the Spirit, who is also specified in context. Consequently, this text contributes significantly to the πίστις Χριστοῦ debate even though its contributions might not be immediately apparent (356).
In this article he interacts extensively with the literature on Galatians and it is another worthwhile contribution to this debate, which seems to have become of extreme interest to him. By my count this is the eighth publication in which he has spoken to this issue. I have updated my bibliography with this article and another. Happy reading!
Campbell, Douglas A. “2 Corinthians 4:13: Evidence in Paul That Christ Believes.” Journal of Biblical Literature. 128:2 (2009): 337-356.