I can barely understand what you are trying to write? The 300mm 2.8 is way out of my budget, but the F4 is affordable and a good lens. As I've read other places, the 300mm F4 combined with a 1.4x extender will work ok, but might not work with the 2.0x? I have a 5D Mark II
I am sorry if i expressedm me unprecisely. I speek from my own expierience, we were in south africa for wildlife shooting 4 weeks.
if i look at the costs we had to pay for this trip, 4000$ for the aquisition of the 300 2.8 was not the major expense. this it if course different if you shoot wildlife next to your home if tehre is some (i dont know where you live)
After such a trip you can resell such a lense if it was a one time use, but i kept it because i hope to go additional times. Yes i would have liked to have a 600mm lens, but i didnt bye one for the same reason as you. if you consider a 70-200 ii its already hald the price, then the difference becomes even smaller compared to the costs of the trip.
the disadvantage of the "big" glasses it that you cant leave them allone in third world countries, means you lug the equipment even on a city walk.
If you need more reach than you can afford with your FF camera, maybe a second crop camera is an option, a 50d is available cheaply and has the best AF below a 7d.
because you reported that you tried a 600 ii i assumed that you have a dream and maybe oyou may be working at a solution to make this dream true, in ignorance of your "budget". Thats why i wrote about the 300 2.8 as a more affordable compromise :-)