Hm. My best tank of gas was on hilly terrain between my house in Portland and my Grandparent's in John Day... There are several passes over 3k feet, and 2 over 5k feet. Their house sits 3k feet higher than mine.
Then I drove HOME from there and got an even better tank. I think hilly is actually okay for MPG's if you're not doing short trips and if it's all highway. The key to MY MPG's was lots of engine-on coasting down hills, sometimes in neutral, sometimes using compression braking [EDIT: Probably a lot harder in your automatic :P ].
262,000 miles makes me wonder if you might need a tune-up... And wouldn't 93 versus 91 octane benefit your car? Higher octane should be a good thing.
[EDIT 2: I misread that one. You were saying your car MUST have lost some edge since you are getting fewer MPG's on higher octane. Derpy me.]
Either way, your build threads are awesome and you have nothing but respect from me.
~Matt