A short update:
I got my 50A back from the shop, and lo and behold, the inconsistency was gone. Sigma representative said they adjusted the lens to a standard body, and then returned it. Now the lens was back focusing either when approached from INF or MFD, which is OK from my books as I've tools to correct that. So Sigma dock it is, and 20 mins of adjustments later, I was ready for real life testing.
In the 300 photos I took last evening, I didn't see focus errors, and got a very good keeper ratio. The lens is now working, and actually seems to be sharp enough that I actually need to decrease camera sharpening preset for JPEGs.
EDIT: I'll have to say that this lens rocks! The background blur stopped down is a definite improvement over Canon's 50/1.4!
Yep, it's great idea of Sigma's...make an expensive device to get customers to do their work for them.
You pay Sigma to spend you time correcting their sloppy manufacturing errors. Maybe Ford should make cars which only drive properly after you buy their laptop software to interface with it....