Steve.. I'm not considering a replacement for my 800mm lens. I used to shoot a lot of motorsport and in particular, motor cycles. Last year, I suffered a stroke, and while I am now in pretty good condition, I dont bother with motor sport any more.
I'm located in Queensland in Australia. The only event I would like to shoot would be the Motor Cycle Grand Prix, but,thats 1,200 miles from me. I wanted to go to see it this year, but, the hassle of obtaining passes is too much now.It would probably take 3 days drive to get there as well. Normaly 2 days, but, maybe 3 allowing for the fact I'm not as fit as I used to be.
I have a 500mm AF-I that I purchased years ago. Its the pre-runner to the 500mm AF-S. I have a TC14E and TC17E, so, that gets me back to 800mm if required. Of course, an 800mm f5.6 and TC14E will give 1,120mm as well. The 500mm and TC14E work well.