Best of 15.6 and worst at 22. Mine is a 2013 but the newer ones apparently are slightly better on fuel. The new engines have variable exhaust and intake timing. My one only has variable intake.
They run fine on the lower octane fuels so outback fuel shouldnt be to much of a problem.

Yes there is a little bit of gear out there for them not quite as much as for the y61 but you can wrap them in barwork like most others just manufacturers are limited.

You will most likely need to stay with 18" wheels. These things have massive brakes hence 18s. You might get away with a 17 but it would need a huge offset and I think this is unlikely. Remember you can put 35s on these legally.