to: Drizzt321
f/8 is for me not a problem, because I also have the EF 800mm f5.6 L IS USM and the EF 1200mm f/5,6 L USM.
400 F2.8 IS II with 1.4x TC vs 600 F4 IS II = The 400 F2.8 IS II perform very good without TC and good with the 1.4X III TC, but I recommend the 600 f/4, because it is sharper and is focussing faster.
400 F2.8 IS II with 2x TC vs 600 F4 IS II with 1.4x TC = I recommend the 600 F4 IS II with 1.4x, because the image quality with th 1.4x III TC is better than with 2x III TC.
600 F4 IS II with 2x TC (taped) with 1DX = net tested yet