As the price of all gas rises the differential between grades drops as a percentage of the overall cost. What was a 5% difference at $.10 /gallon premium vs. midgrade on $2.00/gallon gas is now only a 2.5% difference. It makes less sense than ever to use lower grade gas , as long as there is only a $.10 -.20 per gallon spread.

I haven't, nor expect to, try this on my Boxster, as it is not a daily driver, I don't care what the gas costs. If I can't afford the premium, the car stays in the garage, but then again, maybe we'd just skip the evening out meal that costs a fill up of premium anyway.
But I digress.
I burn nearly three tanks per week in my daily driver, due to commute distance. It is a CX7 Mazda with 2.3 Turbo engine, 6 spd auto, FWD. Because of fill up frequency I think I rule out most of the questions regarding the previous post on 10 tanks, 10 tanks of premium/mid grade.
I have found very similar results on my CX7, doing 2 weeks of mid grade and back to two weeks of premium. 1125 miles of mid grade - 19.87 MPG, avg. $3.22- $0.1621 per mile. 1141 miles of premium - 21. 22 MPG, avg $3.32 - $0.1565 per mile. NOTE: less than 1 cent per mile difference!!
Again, this is NOT a Porsche, but verifies the previous test in a BMW, so it stands to reason it will also be so in the Porsche, to a similar degree.
Add to this, the car is SUPPOSED to get premium gas. Does it make sense to risk damage to your VERY expensive Porsche engine to THINK you're saving a couple of bucks, when, in fact, you are probably losing money on the proposition?

It's the other way around PSL . No disrespect but , the manual tranny gets the better mileage and 0-60 times.