I have a problem with my wrists which makes handholding problematic at times, and even my walking about combo of 100d and 18-200mm is occasionally too much to hold, so any slight weight saving I can get is going to be crucial.
Tripod and monopod are also going to be pretty difficult to use because of my main planned shooting location being from either a ship or a small zodiac dinghy, and given that on the ship there'll be too much engine vibration and on the zodiac erecting any kind of support is asking for total disaster, not to mention there's a lot of rocking I may have to just take the savings where I can as prior trips have proven that 400mm just isn't enough, and manually focusing with an extender doesn't have great results for me in those time sensitive and rocky conditions anyway. Getting a body that will autofocus at f8 could be one answer, but then any weight savings I make by having the 100-400 +ex and not the 150-600 are totally lost in the body instead.
I have a blackrapid already and love it, but I'm always nervous about using it with total combined weight as heavy as this lens and body will be, as the force is in totally the opposite direction than the tripod screw was designed for. I've also got a holster type affair and am hoping that might be a good way of distributing the weight and should hopefully put less pressure on the camera as my hip will take a lot of the weight.
I do think this is a close to perfect as any lens gets for me though - the price, range and quality look to be at exactly my level and needs, and it's probably too much to ask that it all comes it at a low weight too!