>...I am trying to decide if it was worth upgrading to the 80-400 >just to get the extra 100mm length for birds?
I have this lens, and my avatar to the left was taken with it. It is my "birding" lens. But, I'm not a birder, and for that purpose there are a few things to consider. First, the lens is slow to focus and can hunt for focus sometimes. This is very manageable, and I don't think you would find yourself missing a lot of shots once you learned the lens. But, there is a learning curve, and I do remember that it took me a while to figure out the lens, and how to keep birds, especially birds in flight, in focus. If you got the lens for that purpose, you'd probably find it frustrating at first, but acceptable once you got used to it.
Perhaps a better alternative would be one of the Sigma zooms, either the 150-500 or 50-500. You get another 100mm, always good for birds, and my understanding (not having used either) is that they are faster to focus. If I were buying today, I'd probably get the 150-500, save some money, and get the extra 100mm. These lenses weren't available when I got my 80-400.
That said, I'm not looking to trade my 80-400. It is smaller and lighter than the Sigmas, I'm happy with its performance, and I'm surprised by how much I use the short end of it. When I bought it I figured I'd be using it pretty much like a 400mm lens, but often (sporting events, nature shooting) I find myself using the whole zoom range. If you were looking at the Sigmas, that might weigh in favor of the 50-500, but that lens is even bigger and heavier than the 150-500. The 80-400 isn't a small lens, but for a 400mm lens it is about as portable a lens as you will find. So I'm not recommending against it -- I really like mine -- but just saying you should look at some other alternatives if you want some extra reach.