It does seem like it would be reasonable to cap soak at something like 8-10. Maybe that's good fodder for a house rule. I am not terribly comfortable with the idea of having to build encounters for a party that includes a player with a 10+ soak. My concern is that I would have to ratchet up the difficulty or always include an enemy with a piercing weapon of some kind who is there just to counter that high soak. It would be hard to make such an encounter without risking overkill on the other party members.
This, a hundred times.