If Matthew believed that Jesus' life had been pleasing to God, prior to that dramatic baptism, then the 'Sermon on the Mount' (as a treatise on how to please God) *must* reflect Matthew's implicit testimony about how Jesus lived before his baptism.
I've said this before. I just felt like saying it again.
If anyone wants to investigate further, and publish something along these lines, I'll happily settle for a footnote. Have away!