Have you actually ever used an FX camera in DX mode for sports or wildlife? I found it highly impractical. The viewfinder becomes smaller than even the "tunnel-vision" D3100 viewfinder. If you miss the composition slightly, you're out of luck, as the frame outside of the DX part is not recorded. No big deal for static subjects where you have time to compose, but it is for fast moving ones like sports. So I'm curious how it worked for you. I prefer to crop in post. The only downside is the metering as I mentioned, but that's easy to work around, as I shoot sports in manual mode or spot metering anyway.