I own a 2006 WRX limited wagon. I commute 120 miles when I do go to work which is usually 4 times a week. I recently started using 87 octane which contains up to 10% ethanol. This was after always using 92 octane for 42,000 miles. If you live in the Chicago/Milwaukee area you've been living with this wretched gas for well over 10 years. Don't even get me started on the politics of this and all boutique gas blends, thats for another forum. With 92 octane I was averaging a little over 25 mpg. I'm still averaging very close to that, within 1-2 mpg. I can't say for sure if it really has any mileage effect since construction to Chicago changes daily. I can say that I have never heard any pinging and here's why.
Most all newer engines (within the last 10 years) have knock sensors that actually "listen" to the engine frequency. It is located below the throttle body, near the tranny. It bolts to the block. Knocking will produce a specific frequency that when detected by the knock sensor it sends an electric signal to the ECU/ECM to adjust timing. So fear not your WRX is smarter than you think. Run a tank of lower octane and see for yourself. One tank will not damage your engine. If you want more proof research "subaru knock sensor" they're about $80.
My father is a retired engineer who worked for Mobile. He always told me that there isn't an engine built these days that can't run fine on low octane gas, even with the junk corn in it. He also says that there are enough detergents in any blend of gas you can get to keep an engine clean. So don't buy into the cleaner higher priced gas, it's a lie.
If it's all about performance for you then use the higher octane gas. After all if you do the math it's not a huge difference. My tank holds 15.2 gallons and if 87 octane is $4 and 92 octane is $4.20 then the the difference between a full tank is $3.04.