I would love to have the 400 as well as the 300. I have seen some great results from it and the MK11 id definitely lighter than the MK1. Hope you get on well with it. I wonder if I should have got the 400 so I will looking at your results.
and to take part in the hijack
I tend to turn off IS if I'm panning fast, stuff like close in birds in flight or planes, and more so with a TC. In that case I up the ISO. I know someone who swears by mode 3 IS. I do use it sometimes, for high shutter speed shots, but don't find it the be all and end all.
I expect IS was off for this shot, but I do know I left it on accidentally some of the time. But hey if I can't tell is it really that important? This was ISO 1000, 1/500 sec, F5.6, 300mm f2.8 MK11 with x2TC on the 1Dx. Its almost full frame and a bit softer than I'm happy with. But I think that was my panning speed not the IS.Red Arrows
by Tom W W
, on Flickr