Looking for some forum wisdom here....

Summer is closing in again. I desperately want to get to 500mm, which I experienced yet again a few weeks ago at Horicon Marsh in Wisconsin -- a place teeming with birds, waterfowl and other wildlife. While you can get close enough to the wildlife to view and enjoy, most everything interesting was out of the comfortable range of my 70-300L. Sure, I can crop in, but in doing so I lose detail, sharpness, etc.

A 500mm lens would have changed everything.

The Canon 500 f/4 (mark I or II) is out of the question unless money falls out of the sky into my pocket. So what's a guy to do?

Best I can tell, here are my options, none of which I particularly like:
  • Sigma 50-500 (apparently soft at the long end)
  • Sigma 150-500 (I really want to like this lens, but many reviews say it's very soft past 350mm)
  • Tamron 200-500 (no IS and soft at the long end)
  • Canon 100-400 (older lens and IS, and 100mm short unless you add an extender and lose AF)
  • Canon 400 f/5.6 (no IS and see Canon 100-400)
Am I missing anything? What's your advice? Am I stuck with the Canon 400mm options?

As a related question, I know that I would generally need a shutter of at least 1/800s with the Canon 400 f/5.6 because it has no IS. How would using a monopod change things? I have one but almost never use it. How much could I save in shutter speed by using it?

Thanks in advance,
Brian