Glad to bring awareness and discussion to existing and future Tesla owners. Cucubits, your journey didn't seem that rough and I'm glad they made you whole - you are completely right about that sour taste... that feeling of being duped or ripped off, and powerless to do anything about it. I agree, the drop from 5.2 to 4.2 is very noticeable and exhilarating. Fortunately your vehicle's hardware was already fully compatible and just required the firmware update. If you read any of my previous posts, they had to drop my battery pack and replace several parts to uncork mine (4/2016 build).
For anyone else looking at a used 2016 75D, there is no easy answer. Tesla (purposefully) took away the ability to query the car's computer and pull the attribute indicating if it's uncorked - perf_config. Ignorance and/or apathy will prevent most corked owners from having an uncorked 75D, and I'm sure Tesla banks on this in the long run. If the 4.2 seconds is a deciding factory during your purchase, be prepared to test it, and, if it's corked, be prepared to put in some work to push to have it uncorked.
This community here is extremely knowledgeable and supportive. Many on here played a large role in helping me resolve my issue. If you are reading this and run into issues, you will not be left stranded on these forums.