The Contax G 2/45 Planar is an awesome lens, back in the time of it's introduction it was dubbed the Leica M killer as it supposedly bested anything Leica had at that time and reigned supreme for years as one of the sharpest lenses one could buy (so I'm a bit surprised of the OPs findings).
I use the Kiwi adapter which works nice but as both the adapter and the lens have a little play in their focus mechanism there's some play while focusing the lens, your overshooting both ways to nail the focus which isn't perfect but the IQ makes you forget this (non) issue very quickly. I think the much more expensive Metabones adapter works better, there's also an AF adapter available on eBay.
Here's a couple of images of the lens mounted on the a7R (taken with the iPhone 5):
https://farm4.staticflickr.com/3779/11072246366_d923fc3e91_b.jpgAnd of course a couple of images taken with the 2/45 on that same a7R in Spain
And an OOC JPG in APS-C crop mode of my beloved Nikkor 50/1.2 Ai-S, another great lens