I agree that you should be taking your car to the dealer.
I disagree that the described throttle behavior is normal. Subarus are known to be touchy with an exaggerated throttle curve, but they do NOT bog then, after letting up on the throttle, accelerate like crazy. That makes no sense! Sounds like a possible Throttle position sensor or even a CVT issue.
Two points re: your MPG:
1. The wording of your posts leads me to believe that you weren't always getting such bad gas mileage. So, what's changed, if anything (e.g., commute change to all city, lots of idling time/warming your car for extended periods of time before driving off, etc.)
2. What kind of change are we talking about? 15 --> 12? 25 --> 12? Big difference there and a major data point we're missing to make any sort or recommendation.
3. What are you seeing for boost during the strange throttle response behavior? Are you seeing full boost while at the lower than expected RPM that remains after you let off the throttle, with its resultant increase in RPM, or is it truly bogging down initially?