A 250mm zoom isn't going to give you any more of a noticable difference than the 200mm, I don't think. You'll probably need to go to a 400mm or 500mm for a real noticable difference. I have a Tamron 70-300mm, and find myself wanting more zoom too, even though I love this lens. I don't notice a big difference between the 200mm and 300mm length on this lens. Of course there's some difference, but is it enough? I'm going to the NC Zoo next Friday, and will see how the 300mm does for me. NC Zoo advertises itself as being one of the largest "natural habitat" zoos in the country, so I'm sure I'll find out if 300mm is enough zoom.
I'm really looking forward to this trip to the zoo. Last time we went was almost 30 yrs. ago. It's changed a lot since then, I'm sure. I didn't have a camera at all, back then. I'm hoping to take a lot of pics this time. My wife said that this morning on tv, the weatherman said something about a possible hurricane this next week! That would be just my luck! Ruin a trip I've waited almost 30 yrs. for! I've got my fingers crossed.............