Your mileage; based upon what you've said... is about what can be expected for those conditions.
Think of these bikes as if they're big old barn doors... The faster you want to push them down the road; the more it costs.
One member; Texas, did a lot of experimenting with his bike, and he found ways to squeeze out the best possible mileage from it.
He's got a 2014; but the principles will still apply. Perhaps you should PM him...