There has been some confusion with both of your questions. All models have height adjustable driver's seat (not your question) and no models offer a height adjustable passenger seat. An option I use when I have a passenger who is sensitive to the difference: I lower mine a bit and offer the a cushion (I got a perfect fit and comfy one at Aldi market) to match our heights.
As for the break-in period, that doesn't necessarily have a relationship to fuel economy. The sort of slight difference that people sometimes notice (never the case for me across various new cars) shouldn't exceed maybe 10%, whereas what you saw was at least 50%! So after doing the test recommended by "J", if you are not close to the numbers offered on the window sticker this should be brought to the attention of the service department, maybe when you have --if they offer-- a first check at 1000 miles or 1 month.