The so-called "efficiency package" is a package of software/firmware that gets changed every year or so it seems. This time, since there were also changes to the EPA testing regiment for 2024 to make EPA-stated ranges more realistic, this new testing approach was done with these efficiency package changes in place. One reduction was made.
The cars, however, are fundamentally the same, and the "efficiency package" changes are not particularly influential on the EPA-stated range. The big influence on the EPA range was the new EPA testing. In my view, all this was for the better, as Tesla owners know that we typically were not coming close to the EPA stated range.
I've been watching this fairly closely as I have an inbound 2024 MSP. The new 2024 EPA stated range for the Model S Plaid is 359 miles. I recently watched Out of Spec's range test on their 2022 MSP, conducted as a 70 mile per hour highway test (highway testing is hard for EV's due to wind resistance) and the result was 367 miles. So I'd say this new EPA approach is going to yield a closer result to real-world. All for the best. BTW, most people never got the EPA mileage figures for ICE cars either, so this realism is welcome I think.
All that said, the vehicle hardware is the same. Software changed (and will continue to change, which I like about Tesla) and the testing methodology changed.
I'm not familiar with the shudder problem you mentioned. I tested 3 or 4 different MX's before deciding to order MS (mostly because I don't need that big of a vehicle) and I didn't experience a shudder. But Tesla hardware changes come all the time, and will continue to. You will likely never time it perfectly. Buy whenever you're comfortable.