The problem is that you already know that you have the 5.8 firmware, and you expect the car to feel different, so you honestly believe that it is much different (and worse). Perhaps it is noticeably different, and you just have a keenly tuned sense for this, or it could be that you are just "sensing" what you expect to feel.
That is the whole point of a double-blind test. Neither you or the test administrator knows which car is which, and you drive both of them back to back, and you privately record your impressions the handling of each car. I think that a pretty high percentage of people would not be able to tell the difference under those conditions.
This reminds me of the old audiophile debates about speaker wire and expensive interconnect cables, where "golden ear" audiophiles could write chapter and verse about the "improved clarity and imaging" and the "more immersive soundstage" that they could hear due to some particular product. Double-blind tests usually shot most of those claims down.
Even so, I am looking forward to Tesla giving us more control over the settings, or letting us opt out of the new behavior in the future.