Based on your input here, I'd rent the Sony/Zeiss 24-70/2.8 again and test it against the Tamron you already own. It should outperform the Tamron in most cases, but only you can determine that based on your requirements. I've found that methodology to be the only way to really figure this stuff out ... and often the differences are far more obvious than speculating based on other's opinions ... and sometimes they are not.
If you are truly getting more mental about IQ, it may be time to move up to the lower end of Medium Format capture. As good as it is, you can only go so far with a small format capture before hitting the wall. Depending on your actual requirements and applications, throwing more and more cash at that wall in a search for higher IQ tends to eventually become be an exercise in futility. :deadhorse:
-Marc
(BTW, I own two A900s and all of the Sony/Zeiss FF AF lenses, as well as the 70-200/2.8 APO, and while excellent for my wedding photography, wouldn't even think of using any of it for landscape, macro, or other commercial work).