Well if you DO manage to get the spell off on your nice pet roc, might I sugest going for broke and attempting to add strong jaw as a potential permanency spell? If not, just having this thing on the roc when needed will be a huge boon. When your roc hits gigantic, this thing doubles the damage of its natural weapons.
Side note, you cannot have both enlarge person and Animal growth on the same target.
Multiple magical effects that increase size do not stack.
Now my question. Does strong jaw stack with improved natural weapon? Do they stack with Animal growth. I dont see why they wouldnt, but if it works, you could have that rocs attacks count as colossal, then doubled. you are looking at 6d6 for the claws and 6d8 for the bite. I think I got that right, the two abilities are kinda confusing taken together.
Personally I would take the Deinonychus, but that doesnt fly, and is a size smaller, but it has toe claws and pounce ^.^ the thing is scarry powerful, and I never realized about animal growth! Not what your looking for, just what I would look at.
Animal Growth is only similar to enlarge person in that they both increase the size of the target. The power level is radically different. Let me put it this way: Why is Beast Shape III (another 5th level wizard spell) not on the permanency list? Animal Growth has more in common with Beast Shape than it does Enlarge.
Anyhow, good luck on trying to convince your GM.
- Gauss