A week into the K-x adventure. Its size is mighty close to the Lumix GH1, though for the most part lenses are still larger. Not all that much difference between the 45(50)-200 zooms, but crop factors make a difference in final image size, so it's actually a 90-400 vs 75-300 in 35mm terms. Going to the Pentax 55-300 would tilt compactness back to the micro-43 camp, certainly. The elder Sigma 17-70 is massive compared to the Lumix 14-45, but really I like the 17-70 in all other respects. This is my 2nd copy after enjoying it on my A200 in 2009, so I knew I'd like it; in fact, somewhat to my surprise I prefer it to the Pentax 16-45, 18-55 and even 18-135 - so I'm glad it's back even at 450 grams.
I have the K-x set to auto-iso up to 5000, and its straight-from-camera images are similar to what I'm used to at 1600. Truly a step up in sensor quality! Here's a torture test at iso5000 - Raw image, lightly cropped but otherwise unprocessed:
This feels like a lot of dynamic range and reasonable color noise in the shadows for such a setting, and since it's a Raw image I could clean this up if I felt the need. Wow.