How much difference does running premium make? Does it depend on the engine?
It depends very much on the engine. If it runs a high enough effective compression ratio, and
is new enough to use a knock sensor, and
is configured to run as much ignition advance as possible without knocking, you can get some respectable gains from high octane gas.
I don't see how running a turbo would help, either. For the best mileage, you won't be running high enough revs to have the turbo kick in.
That depends on how the systems is tuned. You can
run a relatively small turbo that will make boost at low rpms (and just get in the way at higher rpms) and allow you to run cruise just above idle with the throttle wide open, as long as you run really tall gearing too. Not exactly a formula for good acceleration, but it would give good mileage.
It would also tend to put a lot of strain on the engine by running very high cylinder pressures and low oil pressure (unless you went to a higher volume oil pump too). This is the kind of running diesels are built for.