Got my MX back, MCU was replaced with all the expected components, and Steam works perfectly fine now.
It might be just a matter of perception, but it does “feel” snappier as well. They installed the same firmware that I had on the prior MCU, so it’s a direct comparison other than the hardware. Considering the extra RAM, I can see how that might make a difference.
They still charged me for the part, so I’m still going to try and pursue that further, but I don’t really expect anything in my favor given Tesla’s shitty track-record in customer satisfaction, so far.
Overall, I am happy I did it. 8GB vs 16GB of RAM might only be the difference between having Steam or not TODAY. But I’d like to think I just gave myself more longevity in future software updates and features that would require the same headroom.