[Chilliman]

I just recently bought the 100-400L IS and was almost buying the 1x4 tc to go with it, Handholding it with the IS great but losing the AF or IQ did not appeal to me, fiddling about trying to manual focus is not what I wanted while handholding. Other reason is folk saying you lose IQ with one, well if I am spending almost a grand on a quality bit of glass why do I want to degrade that IQ by spending a extra 180 quid on a TC for a few extra metres. If 400mm is not a long enough reach then maybe a longer lens would be better and retain the IQ.
As for battery packs, do they help steady the camera a little against camera shake? Adding weight to the camera in the form of a battery pack seems to me making things hard for yourself, Personally I'd rather just carry spare batt's in my bag. A camera and long lens is heavy enough to carry around on it's own without adding 2 extra batts etc.

You'll love the 100-400 by btw, I've just posted some pics in the Photo Sharing section under Edinburgh Zoo all taken with the above lens. I've seen better but then I'm still learning this game.