Sending this from Australia. I am surprised that your 'top tier' gas is 87. Our 2005 Forester manual says the car is designed for 90 octane. Our fuel pumps are 91 (unleaded), 94 (alcohol mix - cheapest fuel), 95 and 98. I always use 98 and the car still pings with pre-ignition noise. Your model must have a lower compression ratio. In regard to fuel usage 8 litre/100km on a high speed trip and 13-14 litres/100km around town even when driven with light throttle.IMO you should be doing better. On road trips, mostly Interstate, our 2015 turns in 32mpg on flat terrain with CC set to 65. In daily around-town mixed driving, it averages 25.
The tires are inflated to 32, and they are aligned. We accelerate gently. We maintain steady speeds. We use Top Tier gas (87 octane). And really, that’s about it.
There’s something wrong with your car, or with your driving.