I can't take any comparison shots right now to back this up, Tessfully has the K-5 down south with her. I'm left with the K-x and K20D. (Every body say "awe, how sweet!". ) SO just a few comments. One changing the ISO in good light is not what the extended DR is about. Where you notice the difference is when you are shooting in low light. And the difference is not so much in noise, but in the rendering of natural, rich looking tones in the bottom end of the histogram. We shoot every day, where the K-5 really shines is the cloudy days under the forest canopy. The fact that there are more gradations in the high end than the low end of the spectrum, for reasons I don't completely understand, something about bit order etc... the 14 bit gives you many more gradations in the low end, and when you transfer to jpeg, you get a much more natural rendition, because you are using real gradations as captured by the camera instead of making up fill that ends up causing banding.
That being said, our current rule is first out the door uses the K-5, If she's first I use the K20D and if I'm first she uses the K-x. A while ago the K-x was dropped and we had two shooters and one camera. This is not a recipe for marital bliss. We decided to keep my K20D when we bought the K-x as a back up. At this point we are considering selling the K20D and K-x for another K-5. I can't even explain exactly why that is true. It just is. But I'm sure it a combination... better low light performance means more accurate focussing accuracy and speed with slower lenses. WIth an F2.4 DA35 on the K-5 focus lock is very quick. I can't say it does much for my f4 DA* 60-250 but it seems to really help a faster lens. The better lower half of the histogram is part of it. The better recovery from highlights is also right up there on the list. But more than that it's just the confidence. We shoot a lot of images, and if every thing isn't just perfect, you have more chance of being able to save an image with a K-5.
When the camera gets back, I'd do a test, but I don't know how I'd measure most of this stuff.