I have experimented with mid-grade versus premium, comparing gas mileage on identical drives (104 mile daily commute) numerous times on my 9-5. The results are always the same. I gain sufficient fuel mileage when using premium to more than make up for the higher purchase price.
On my wife's 9-3SC 2.0T the fuel mileage with premium is staggering. We acieved 33.4 mpg on a tank of premium when driving 80mph thru the mountains of West Virginia. Don't try this at home, we were only keeping up with the flow of traffic.
Mid-grade is a false efficiency and I won't even try 87 octane.
I also drowe +1 year with low octane (95) gasoline, until june this year when I changed to 98 octane. What a difference! With 98 the throttle response is a lot better and torque at low RPMs is noticeably better.
I didn't have ANY problems with 95, but then again I didn't know what I was missing. Now the car works like it's supposed to. You people should give it a shot. For me upgrading the fuel made a difference.