I've also had mixed experiences with this lens on my A7 so far.
First, I bought this lens in a Pentax K mount and used it with a cheap K&F Concept adapter on the Sony. Apart from the barrel distortion, the results were really good. Pretty good corners at f11 IMO.
To deal with the distortion, I jotted down the amount of shift for every shot and used this data in the Capture One Lens Corrections tool's "movements" tab along with the Distortion slider set to 24% (there is no C1 profile for this lens alas). The results were perfect for stitching, you can find a full resolution layered tiff (three images, shifted 12mm left and right) here:
https://dl.dropboxusercontent.com/u/3949609/Untitled_Panorama4.tif
View attachment 125384
There was one problem though: on the Pentax K to Sony E adapter, the camera and lens mounts were rotated about 4 degrees against each other. This is a non issue for normal lenses, but with a shift lens used for stitching, it means that a shift to the left also shifts up a little bit, and a right shift also shifts down a little bit.
So, I exchanged the Pentax mount lens with the more expensive E mount version. This version was optically as good as the first one, but it had massive internal reflections inside the lens tube, which made it practically unusable. Example here:
View attachment 125383
So, at the moment I'm not sure what to do with this lens. I would have to paint or line the lens tube with flocking material to make it useable.