[edwardmagichands]

Based on my short knowledge(if you say "shut up" because of this, then, well....), gasoline detergents are not so effective to clean up the intake valves in these days' *direct injection* engines since the gasoline doesn't go through the intake valves.

Also N52 needs minimum 87 while N54 needs minimum 89 due to turbo charging which means hotter air-fuel mixture inside of the cylinders which leads to premature ignition. Higher octane will prevent this.
To lower the temperature of the air-fuel mixture inside of the cylinder, ECU adds more fuel into the air and that leads to bad fuel economy. By using higher octane gasoline, less fuel is needed to prevent premature ignition and it will help a bit of fuel economy.

That's why mileages of many turbo charged engines don't match to the manufacturers advertised numbers if you drive hard - well.. I mean much worse than NA engines.

However, consensus(?) is that the difference is not that much and that's why, if you don't care about power when you need it and care more about $120 a year fuel cost, it's OK to use minimum recommended grade fuel.

my 2 cents.

By the way, in Canada, if you drive 10,000km a year and my 135's mileage is 16L/100km, cost difference between 87 vs 91 is about CAD $0.17/L, so it's about CAD $272 = US$208.