Brant, I can see from the EXIF on Flickr you're using lightroom. Try the AI noise reduction on that first shot and prepare to be amazed at how much detail you can squeeze out of the image.

As for 100-500mm+1.4x vs. this lens... no idea. But as this can ALSO take an extender, if it's reach you're after, this one gets more of it. Provided it's not overly soft, that should mean 200-800 wins. Especially if the extender works over the full range, not just 300-500. Being able to zoom out to 280mm instead of 420mm should make finding things far easier.