Stimilated by this thread, I took some shots today with the FA*85 and the teleconverter on the K-1.
In full frame mode, wide open, it vignettes a little in the corners. The effect is more pronounced at infinity focus than MFD. I won't bore you with photos of the sky unless pressed.
Stopped down, the vignetting is not as noticeable, but the image quality in the corners falls off dramatically. If the image below doesn't convince you, click on it to view a larger version in Flickr.
Although the FA*85 exhibits a bit of glow wide open, when you add the teleconverter it turns into something like a Helios!