Unfortunately, your old 80-400mm will not work faster with the D7100. I bought one of the first wave and tried forever to get used to it but I missed so many shots of flying birds that I sold if for half of what I paid for. That says a lot for a lens where people don't even want to pay half of what it sells for new. Mine by the way, was in mint condition and I sold it to a fellow nikonian for $900. I don't know about the new one but they claim that it focus faster. I have an 18-300mm coupled with my D7100 and it focus very fast. Good luck with your choice!