Looks like I may just go for Canon 300mm f/4 IS with 1.4x TC. Not as much reach as I wanted, but at least it should be more portable.
I have the 300L f/4 IS and I've been very happy with it on my 5D2. Fast AF, good IQ, and not excessively heavy. The IS is awesome, and is the main reason I picked this lens over the 400L f/5.6. I just
got the 1.4 x mk3 TC, taking advantage of the current rebates. This gets me 420mm at f/5.6 and
IS for not a huge amount more $$$ than the 400 would have been.
I am on a business trip to Japan right now and I didn't want to lug along the 300, so I've been trying out the TC with my 70-200L f/4. That combination seems to be working pretty well, although I haven't looked closely at the results off camera yet. I took a bunch of shots of formula cars at the Motegi racing circuit, and the AF seemed to track pretty well even with the TC. So I'm hopeful that the combination of 24-105, 70-200 and 1.4 TC will make a nice light and compact kit for casual travel when I don't want to carry my full set of gear.
I'm looking forward to trying out the 300 plus 1.4 TC combination in earnest when I get home. I only had time for a few quick shots of birds at a feeder before I left, and I didn't nail the focus. I did do my homework before buying, and came to the conclusion that the IQ of the 300/1.4 TC pairing was close enough to the 100-400L and the 400L f/5.6 to make this combination viable.
Like you, I have been vaguely evaluating 400 f/4 (or equivalent) options. The cheapest Canon solution would probably be a used copy of the 400 DO, which can be had for less crazy money than things like the 300L/f2.8 with TC, for example. But I'm thinking my best option will be to pick up a 7D to get the reach I want. The 7D is far less expensive than most fast 400 options, and has an AF system that is up to the demands of bird photography which the 5D2 really isn't. As a bonus I get a backup body as well. Until then I am stuck with borrowing my daughter's T3i when I can get away with it.