Before getting the DA*300 I rented a Sigma 50-500 and used it for a couple of days at Magee Marsh during the spring Warbler migration. My decision was between these two lenses, I chose the Pentax as it is faster and most of the birds I'm after are are in trees in wooded areas, so it's sometimes pretty dark in the forest. The Sigma struggled in some areas. That 500mm is nice if you have enough light, but the Pentax is faster and has excellent IQ. The autofocus could be faster and it would be even better. One thing a lot of the better bird photographers says is get closer. It's true but very hard. I don't think I've helped you much, but technique is probably as important as the lens, and lens selection may be based on the habitats you shoot in, you have a lot more light shooting ducks on a lake than woodpeckers in a dense forest. Here's a few examples, most of my bird photos are cropped, usually a lot.
With the DA*300, the large barred owl was about 50 feet away:
With the DA*300, This northern saw-whet owl was about 20-30 feet away, a much smaller owl and cropped quite a bit:
DA*300: This Golden-Winged Warbler was so close I couldn't fit her in the frame:
This Ruby-Crowned Kinglet was shot with the DA 55-300, the bird was only about 10 feet away.
Da* 300: Palm Warbler, heavy crop
I have a lot of examples on my Flickr site in albums: Warblers, Birds, Owls