Victor, the focus action is not only very smooth but also very tight and well-damped. By "tight" I mean no play; it turns very easily. It has a very nice focus action, but the aperture ring was a bit loose. I opened it up and the mount was shimmed with bits of what appears to be that adhesive-backed aluminum tape used in ductwork.
The average shim thickness was about .65mm(by no means consistent, was ±.12mm or so) and it seems that what is needed is about .25mm of shim to make it fit right, leaving about .05mm of play. If I had a second aperture-detent plate to drop in it would be near perfect.
On top of that, the mount itself has been filed down unevenly -- and I mean filed by hand.
I'm going to get the mount on a lathe and get the surface that contacts the frame of the lens turned down just enough to ensure that it is flat and parallel with the surface that contacts the body. At the same time I can probably have the edge relieved to clear the aperture ring rather than shimming it.
Addressing these issues ought to get the lens about .45-.55mm closer to the sensor plane, and I can fine-tune infinity focus from there following some instructions that I found elsewhere that allows me to effectively adjust the fore-aft location of the moving elements when I hit the infinity stop.
For now I just have it assembled with zero clearance at the aperture ring, so it is essentially locked wide open. I'll snap a couple more shots when I have daylight and see where that leaves me.
I'm no expert on optics -- just an enthusiastic hobbyist -- but I would think that having the optical axes of the lens and the camera misaligned would not affect edge sharpness but would instead cause the point of sharp focus to be closer or further away than at the center of the frame. So I'm assuming that the softening at the edges is inherent to the design, but the way I intend to use the lens I can get by with it. I'm thinking nature and maybe the odd portrait shot where the edges are far out of focus anyway.
The offer of a loan is very generous, thank you; I'm in NorCal, Sunnyvale specifically. Where are you located? More importantly, what mount does the lens have? Currently I am only equipped to handle A-mount, T-mount, and MD/MC lenses on my a900. If this is workable I'd be happy to do side-by-side shots, using my battleship tripod, MLU, cable, etc. and a better target.
If we are ever in the same area you are absolutely welcome to shoot my a900 and any lenses you want but with most all of my eggs in this one basket I'm not in a position to loan long-distance. I know where you can rent one though... try alphalensrental.com. Matt seems like a good guy, I have enjoyed doing business with him. Last I contacted him about 2-1/2 weeks ago he was a day or two away from moving to the West Coast and he still hasn't updated the status of gear on his site, so he may have it in stock.