Like outsider, I also shoot mostly near wide open (I have 4 primes (both MF and AF) faster than f/2). The main point is that we both get different (and in many ways better looking) results with the K-x. Simply by changing bodies.
FF is only part of the issue here. Images from the K-5 also have a more "muted" look. Outsider obviously knows what he's doing in terms of shooting.
I like Pentax. And I can clearly see from online tests that the K-5 has better high-ISO noise performance than any other APS-C DSLR out there - a feature which I want. But this doesn't mean that the K-5's images look better overall. In some (perhaps many?) instances I like the images from the K-x (and the K-r, when I've used it) better. A close look at the dpreview studio comparison shots reveal differences (between the K-5 and K-r or K-x) similar to what we've been discussing here.
As I've already suggested, there are a number of factors at work here, and FF is only part of the story.
Outsider, I suggest you try this technique that I've used:
Put the K-5 in MF mode and continuous 7 fps shooting mode (make sure you have the updated firmware that allows more RAW images in the buffer). Then slowly turn the focus ring as you fire away. You'll get one or two shots with nice focus.
I can already tell you what you'll get. You'll get an image that may be slightly better focused on the eyeball, but all the other image characteristics will be similar to what you've previously noticed.
I'm wondering if most posters here have not owned these bodies and/or don't use the better, faster primes that make these differences obvious.