1D, 1DmkII, 1DmkIII, and 1DmkIV owners use a 200mm, or 300mm instead of a 400mm or a 500mm (etc.) with a 1.4 or a 2X extender to shoot fast action, birds, cars, sports, etc?
The reason I ask is, I am about to get a 1DmkIV, and I have a 100-400mm L IS zoom that I have had for around 4 years, and I wonder if I couldn't get a more sharp shot if I used something other than the 100-400mm. I am not against getting a 500mm of some kind, but to be honest, I am not that familiar with those, and some are out of my price range, period, a more suitable price would get me into one though.
I love my 100-400mm, but from what I understand even with a 1D series camera, I will have trouble with the 5.6 aperture getting me the shots I need to get in birds in flight. (BIF)
I use the Canon T2i, the Canon 100-400mm L IS as of now... I want to be able to use an extender when I upgrade.
I have the Kenko1.4X Teleplus Pro 300 and the Kenko C-AF 2X Teleplus MC4 extenders now, I am not opposed to getting a Canon extender if need be... (The 2X extender is only good for manual focusing for the camera I have now)
I can't really use ANY of the extenders I have now because they slow it down and make it unusable for BIF, I can however use it for static distance shots, with a loss of quality, but I still am able to get that shot if needed...
So what I am really asking is, I need a tried and true combination, are there any of you that actually use an extender, and a 1D series that are able to get keepers in BIF shots?
I knew a professional photographer that had a 1DmkIII that used a Canon 1.4 extender with I believe, a Canon 500mm, (I don't know which) and he takes perfect BIF shots, but I don't really read that here on this site for some reason. (He also had money to spend, I don't have his money) 
Please help me out if you can, I am about to be needing to make the decision soon of the camera, extender(?), and the lens.
I may have to get a Canon 400mm PRIME lens, that might get me to a place that I wouldn't need the extender so much, if at all, that would be fine, but I see so many lenses out there to choose from, and I am confused as to which are what would be best for me.
The lenses that I know would prevent me from needing an extender are really out of my price range, I could, but I don't want to spend $5,000. to $12,000. that I would need to. (I know, just like ALL of us!) 
Hopefully someone could come up with a little less expensive route for me to take, I'm all ears. I might have high aspirations, but I am still not in the money... 
Thank you, I appreciate any and all help that you can give.
Randy





