I got the Tokina 11-16mm (Mark I) because I wanted a wide lens I could use on both the GH3 and BMCC (if it ever comes out). I just sold the 14mm with converter on Craigslist and I thought I should do a quick test before the dude comes and picks it up. The converter is the official Panasonic DMW-GWC1 designed for the 14mm pancake.

I did three comparisons each with the lenses at f/2.8 and f/5.6. The uncropped shots are resized, but I only wanted to demonstrate the barrel distortion and vignetting with those. There are also 100% crops of each. All shots were done with the wide converter on the 14mm (sorry, forgot label it properly), and with the Tokina at it's widest setting. I used RAW on the GH2 for all shots.

As expected, the Panny combo had worse vignetting and barrel distortion than the Tokina. The GH2 corrects the distortion for the 14mm, but it can't do anything about the wide converter. At F/2.8, the Panny combo was clearly sharper than the Tokina which had some noticeable coma. At F/5.6, the sharpness is about even in the center, but the Tokina slightly beats out the Panny combo at the edges. You can also notice the purple fringing is pretty bad on the Tokina at the edge of that white book.