If only Canon had a 400mm f/4 prime (not the DO). I tend to have my 1.4x welded to my 300 f/2.8 and it will take the 2x as well, for an IQ hit. I have used that combination successfully on my 7D for a few years, although I've tended to avoid the 2x, so don't have a vast amount of experience with it. Now that I have the 5D MkIII, I do lose the reach, but so far, I haven't missed it as much as I might have thought, due to the improved image quality. Even cropping, the better AF is making a noticeable difference. A pro I know, used to use a 600 f/4 on his 1D MkIII, but he dropped the pair going through a gate. He replaced the 600 with the 500 and said he got better shots, because he wasn't struggling with the weight of the 500 and could use the 1,4x, whereas it was near impossible to control in locations where a tripod wasn't possible when the 1.4x was added to the 600. With that in mind, if you can find one, a 500mm f/4 L IS (original) might be a good bet, the ones I've seen have been just under £5000, although MPB Photographics had a well-used one for under £4000 I think it was. I was sort of tempted, but it was just too cosmetically damaged from the pictures.