That's great advice. Take it. You would have difficulty using that lens for anything but parked planes. Focus is painfully slow. I recall instances where I could hear and feel the lens searching for focus, back and forth, while I waited to take the shot. At some point, my brother-in-law borrowed the lens and I was happy to "forget" to ever get it back.
If planes are your primary focus (heh), consider whether you really care about FX. The 7100 has longer reach, and DX lenses are much easier on the pocketbook. Instead of a D610 (which is a fantastic camera, by the way), you could pick up an 18-300 and have a lot of money left in your pocket. It's not the greatest lens on earth, but it
holds its own and I would rather shoot with it any day over the old 80-400. And, there are many other DX options. In the FX world, I agree the (newer model) 70-300 is a
much better option than the 80-400.