Just a quick question, but what is making you want a monopod with the 400mm f5.6?
When I had this lens I found it to be VERY easy to use handheld down to shutter speeds in the double figures sometimes.
And with the lens being sharp wide open, fast shutter speeds aren't too difficult to come by, especially when teamed up with a 7D which by all accounts performs admirably at high ISO's. The exception being if its a really dull day or your in a dark woods, but a monopods not going to give you a distinct advantage over a good hand holding technique I feel.
A monopod seems neither here nor there to me when teamed up with this lens, for the amount of help its going to be I'd see it as a bigger hindrance having to get the height of the monopod roughly right each time before you want to take a photo.
This however is just my opinion, let me know if you see a noticeable increase in IQ
Ryan