I've been saving up for a long prime to photograph wildlife. Currently I'm using a 70-200mm VR f2.8 with the TC-20iii, on a D7000. I'm looking to eventually get into FX so focal length might be an issue. I know the 600mm and newly release 800mm would give me a long reach, but what about using the 400mm and 1.7TC? Which would give the sharper shots? I've been playing around with taking shots at 200mm and cropping as well as taking them at 400mm uncropped. I think that I lose sharpness when I crop over using the TC. What have others found on this? Is is better to go extra long and not have to crop, or use a TC to get me there? Or is staying shorter and using DX a smarter move than long and FX?