If I were to balance this I'd either reduce its drain value to 2, or add soak dice to its function. While yes it can roll better than increase body, once it can also roll low enough it does not matter and two body has some functions beyond DR and soak, like I don't think armor would help with toxins etc. And its 5 DV is based around a fairly normal roll of increase body. The low risk choice is reduce the DV to 2. The game is balanced around lower soaks so a spell that can boost soaks may change the low of the game. Probably wont much when cast on the mage, bumping their 4 soak to 8 isn't a huge issue, but the monster street sam who already has 11 soak going to 15 may skew things to where DMs may start throwing ludicrous damage at the party so the Troll can be threatened, which works up until they shoot the decker.
I'm not sure I want a fix for ITNW, I do not mind spirits being sort of indestructible, I think its sort of a red herring in spirit balance. I do mind their output. If their damage/utility was more PR2 level threats/help on the high end being really hard to kill would not be so much of an issue, and they would not be out street saming the sam.