I think you've made a good decision. I don't have the 15 yet, but it will be coming in the next month or so. I love my K10D and haven't been given a good enough reason to upgrade.
The K10D will also do larger than 20x30 just fine. The 30x30 I ordered just came in last week and looked incredible.
To see images from the 15 in the gallery, go to the homepage:
PENTAX Photo Gallery, and click on "Image Data" near the bottom of the page, two options will appear: "camera model" and "lens", click lens and select the 15.