The 300mm f4 is a good lens, the 2.8 is superb, but at a superb price There's always the 400mm too...
On the whole prime lenses do the job better than zooms, but they don't have the flexibility.
The canon converters are good, I haven't noticed any loss of image quality using the 1.4, but not sure I'd recommend the 2x unless you have a fast lens to start with. They can slow the focusing down a tiny bit, but it's hardly noticeable.
The 100-400L IS is a decent lens - if you get a good one... It seems to have been around for a while though and may be due for a revamp.
thanks mutt so