I am feeding Eagle Pack also, with such a big puppy you have to be careful not to overfeed protein. Whatever brand you choose, try to not go too much over 23% protein. The protein is what makes them grow in a faster or slower rate. Like greek4 said, it is also important to have a good balance between the protein and energy. But most of the too high energy values are in the food with too much protein. Your dog will get as big as his genetics were meant to make him, but if you can make him grow rather slow and in a even rate he will benefit a lot from that. Both hips, elbows and other joints take a lot of stress when our big babies grow. You also want a nice calcium level, too much can make them grow too fast, too little can make them knockle over, basically it is a fine line.
Even if you find a brand that is a bit more expensive than Iams, see to the whole picture. how much do you need to feed, how will the over all health be on the dog. A real good food has usually way less fillers than the little cheaper, and you can usually tell by how much you have to feed and how much they go poop. Especially when they are out of the baby stage.
Good Luck in finding a food that works for you
Marit