While I don't have your camera (I have 1Ds2) I do have the 300 2.8 as well as 500 4.0, so I might be able to give some persepective.
I'm reasonably strong and 215 pounds, not much fat, so I figured I could use the 500mm hand-held much of the time. In reality it's not fun holding that thing up for long, it's not so much the weight but the fact that it's all out in front of you quite a ways. I thought a lighter lens would complement it well, even if it often had a 1.4 or 2.0 on it much of the time. Owners of the 400 DO will dismiss all negative talk as applying to old models but there were too many instances of talk about funny background blur, I figured I'd go it safe and get the 300 2.8. It is MUCH easier to hand hold than the 500. I never second guess if I don't have the 500 with me, getting some image is better than no image. I have been pleasantly surprised at the performance of the 2.0. Attached is a down-res'd image along with a 100% crop, which also had to be down-res'd. I'm more than happy with this and other images I have with this lens, if you PM me I will send a larger file to you if you wish to see it.
I'm reasonably strong and 215 pounds, not much fat, so I figured I could use the 500mm hand-held much of the time. In reality it's not fun holding that thing up for long, it's not so much the weight but the fact that it's all out in front of you quite a ways. I thought a lighter lens would complement it well, even if it often had a 1.4 or 2.0 on it much of the time. Owners of the 400 DO will dismiss all negative talk as applying to old models but there were too many instances of talk about funny background blur, I figured I'd go it safe and get the 300 2.8. It is MUCH easier to hand hold than the 500. I never second guess if I don't have the 500 with me, getting some image is better than no image. I have been pleasantly surprised at the performance of the 2.0. Attached is a down-res'd image along with a 100% crop, which also had to be down-res'd. I'm more than happy with this and other images I have with this lens, if you PM me I will send a larger file to you if you wish to see it.