Not sure what you're asking - handling? hand holding a 400mm (equiv) lens? Focusing? Canon in particular?
My 2 cents:
I have the Hexanon 200mm f4, so I can only address a few points. At 400mm equiv., focusing can be challenging - meaning there is no such think as 'quick' - shots must be anticipated and pre-focused. Bright conditions will help, of course. In normal daylight the screen was plenty bright even at f4 - your f2.8, so much the better.
I also have the 135mm f3.2, which I prefer to the 200mm. The image shake when hand holding at 200mm is pretty high! Others have noted that a minimum shutter speed at that FL is critical - for me that means 1/300 or more.
Sorry if this is not what you were looking for...still just my 2 cents.
More along the lines if someone has the 200 2.8... I am pretty confident in handholding it, as it weighs nothing compared to what the Sigma 120-300 2.8 weighed... Best practice for long lenses is 2xfocal... So I'm sure I can bump up the ISO to get a 1/400