I decided to pick up a Minolta 100mm Macro to re-visit what I had tried years ago (ie I came to the same conclusion that at the time for a new purchase the Tamron made much more sense - ie the price I paid was grey import and not much about £230)
The cost of the Sony version was IMO excessive for such a lens, and at the time the Minolta 100mm F2.8 (D) was not that easily available and costly too.
I grabbed a copy from a dealer in Japan it's absolutely mint as good as new for just over €200
My conclusions are as follows
Optically the differences are so minor that anyone that has the Tamron need to worry that the Minolta/Sony is better. In some ways the Minolta is in terms of build (just feels more dense), the metal extending barrel v the plastic on the Tamron, focus hold button which is not present on the Tamron
Wide open the Tamron on full frame is slightly better/sharper in the extreme corners (though the copy I have of the Minolta is quite sharp there on flat level, but not as sharp as the Tamron). Where the Minolta pulls ahead a little is in the middle it is a bit better wide open (you wouldn't notice this for most shots). Both are very usable wide open and make for good portrait lenses
The Minolta has slightly better light transmission (ie T stop identical settings in camera it shows a slight increase in actual exposure on the Minolta). CA are similar on both and can be an issue, a little less on the Minolta at times. Both lenses required AF adjustment on the A99/77 - and are slightly off for critical work on the A57 (but not as much)
There might be valid reasons for picking the Minolta or the Sony, I tend to avoid Sony lenses as I don't really like the ultra fine ribs, no doubt the coatings are updated as they appear to be on other lenses. The internal clutch on the Minolta can be useful, v the push/pull on the Tamron -not a huge deal for me. I do prefer the feel of the Minolta v the Tamron, but as far as optics go there is little to gain picking one over the other.
I will probably keep the Minolta and sell the Tamron purely because I like the build better, other than that if anyone is digging around for prices you might find the Tamron a better buy. Looking at DK's 90mm E mount Macro article, I could safely say both lenses wipe the floor for edge performance at least going off his 2 samples. The Tamron has probably the best edge performance I've seen on a macro lens wide open on full frame
300% middle extreme example but shows a slight advantage to the Minolta
Edge performance here the Tamron pulls ahead (200%)