The Egyptians had the idea the soul was made of multiple parts as opposed to a singular entity which was "you." You weren't your soul, you were a composite entity of your body, your body's spirit, your higher spirit, and probably some other parts I'm missing. So the idea of Spock passing on "part" of his soul would seem insane to us Westerners but would seem perfectly natural to Ancient Egyptians.
It's relevant to Data because he's not just Data, he's Data, Soon's body, and the "higher selves" of Lal, Lore, and others.
So, yeah, I have no problem with Data is a "new man."