"Major improvements all around" could refer to the cars as well. Or could just refer to a myriad of changes to the superchargers (faster max rate, more solar use, buffered from grid via power packs, etc.). Hard to tell from a tweet. I thought in a recent (Q1, Q4 2017?) Elon and JB said they had no plans to change the S/X line (including battery packs) this year (2018). I may be mistaken but that was my remembrance. If so, then we would likely not see a change to correspond with using the 2170 cells until at least early 2019. Which would give them another 8+ months to get 2170 cell and pack assembly fully ramped up to not only handle 5000-6000/wk Model 3 needs, but also enough extra for S/X. 2019 sounds more likely for 2170 for S/X to me, personally. But would it be January 2019, March 2019, summer, etc? Very hard to tell.