So my question is does this spell do anything for a huge hippo(druid)? When looking at the quoted section bellow then looking at the table it looks to me like strong Jaw would do nothing for a hippos bite. The hippo already does twice its expected size damage. If it does work it opens the possibility for to hit for 24d8+mod(not sure if its a big deal).<-the idea here is strong jaw and improved vital strike, I am sure other things can be brought to the mix.
(Strong Jaw)
http://www.d20pfsrd.com/magic/all-spells/s/strong-jaw
(quoted section)
Laying a hand upon an allied creature's jaw, claws, tentacles, or other natural weapons, you enhance the power of that creature's natural attacks. Each natural attack that creature makes deals damage as if the creature were two sizes larger than it actually is. "If the creature is already Gargantuan or Colossal-sized, double the amount of damage dealt by each of its natural attacks instead." This spell does not actually change the creature's size; all of its statistics except the amount of damage dealt by its natural attacks remain unchanged.
^could I get another to agree with Wind Chime
:(
I don't see a reason this wouldn't work. The entry for the Hippo doesn't indicate that it's damage is increased in such a way that it wouldn't stack with other sources of damage that effectively increase your size (such as Strong Jaw).
I will say that while Wind Chime stated that 348 damage isn't any more than certain other characters can do, it really depends on the level of optimization of the rest of your party. If one person mega optimizes their character and the rest of the group doesn't, you're going to make your DM's life hell, so pay attention to your group. If they're all pushing the limits, then by all means, go for it. If not, then I'd give pause to this idea personally.