I looked at Geizhals.at :
400/4.5 = 2883€
600/6,3 =4762€
400/2.8 TC = 14499€ (and will never drop)
so, 7645€ vs. 14499€
hm, I don't know if I am practical but I was also looking at 400TC ... mainly becasue of its ability to convert to 560mm and of course, the quality!
I inherited 400/2.8 FL from my partner who is now using 400/4.5 and I must say that 400 FL is amazing lens but very heavy if you are running with it! Last year I was in Chitake Springs, Zimbabwe running / walking with that lens . On the second day I was out of power and switched to 500 PF. 400FL has quite a good VR but after hand-holding it and simply carrying it for two days .. ufff.. the hands are shaking and muscles are in pain. and the tempererature +43C so, I was done.
So, you should keep in mind that you need to use 1/800 at least and mostly you use 1/1000-1/1600sec anyway and probably don't need a tripod. The problem is to get it to position, to get a subject in IVF and to hold a few minutes (even seconds!) It is not the problem of shaken photos becasue with high shutter speed you will probably even will not use VR but the problem is of shaken hands - conmose, recompose, follow the subject etc..
But if I had money I would probably buy 400TC.
I have also 100-400 and 180-600 and was shooting with both. Why not 400/4.5? Because I am taking video footage as well and need to zoom it out! So, 400mm is often too long! And when I was changing from photo to video it was a bit longer (or cropped) so, I always needed to zoom out.
I bought 180-600 also becasue I shoot video. You just need to have a zoom! And I zoomed a lot in my videos.
If you don't shoot video than go for primes! All three lens 400/4.5 , 600/6.3 and 800/6.3 are definitelly excellent! My partner has 400/4.5 adn 800/.6.3. The quality is stunnning.
I must say also a very interestning thing: I noticed that f2.8 was not enough in many ocasions! However, I love to separate my object from background. I need just all the time to keep in mind a DoF table while shooting with 400/2.8 !
Instead with f4.5-5.6 I don't even need to think about it!
The other disadvantage of 400/2.8 is missing shots! It is big, heavy and until I will get into position the animal is gone
or the situation is changed.
I am preparing for my next Africa trip and knowing the locations I just made a plan what I will be shooting and where. I would recommend to do the same. For example, if you are on the Chobe river shooting birds then it will be z8+600 (and not 800 becasue it wil be difficult to hold it for a long time, with 600 you are just better - also to get flying subject into IVF) . If you are in the national park where no off-road permitted than it will be z8+800. If you go for sun-downer than 70-200/2.8 and so on ...
If you are not a self-driver than you need to take in account that there are other people in vehicle and it will be difficult to use 400/2.8 ... you will just turn it and somebody will drop out of the car to the lions ...
So, well.. if you ask me than go for 400/4.5 , 70-200, TCs, 600 and 800. You will see, on Safari you will be using 70-200 and 400 at most ;-)