Has anyone found anything that works to significantly improve gas mileage of the Ascent?
Yes. Let the car drive. Sounds funny, but I'm serious. Somewhat
I've learned 2 things since I had mine (9 month now and almost at 12k):
1. Everyone that says here to give it some time for gas mileage to improve is right! I've seen big improvements in the last months!
2. The car drives itself more efficiently than me. Even though I consider myself a very mild driver, I can not beat the adaptive cruise control (and obviously it's only something for longer drives with mild or no traffic). Even if I feel the braking and accelerating the ACC does is more aggressive than I would do, it gets better mileage.
What blew me away the most was my gas mileage towing. I have a 4000lb pop-up trailer with a high tongue weight, 470lb so it's close to the Ascents limit. Everybody said it'll cut your MPG in half. Not for me, quite the opposite. At that time I was still only getting 19.5 mpg mixed urban and short fwy trips. On our first tow, I set the ACC to 57mph and cruised along. 19.5mpg on a 500m roundtrip! And it tows great!
Since you are looking at a Hybrid I assume towing is of no interest to you.
Since then it's only gotten better. We've gotten 26mpg on Family trips (2 small kids and full car). I even reached 31mpg during a recent solo trip coming back from the Eastern Sierra with driving through LA! What remains bad and I can echo what others have said here. The really, really SOCal urban driving sucks and we still only get around 16mpg.
I doubt the regular Highlander would do any better though. The Hybrid Highlander on the other hand is probably hard to beat, like the others have said as well.