the marine world is a world sort of similar to aviation, boats do not loaf along on a fraction of the available horsepower, they work hard , all the time the boat is moving (picture a 30 foot sea ray at 23 knots) those engines are converted "car engines" they are almost always getting overhauled at around 650 to 700 hours. If your car needed an overhaul after 700 hours (it would be every 2 years based on 6 hours a week. that would not be acceptable.
the computers on many modern cars shows the average speed , i have never reset mine, it sits at 56 km/hr, and that is nowhere near 135 hp all day...
what we are all on about , i think called specific fuel consumption, pounds (or kilos?) of fuel per hour per hp (watt?)
that is the only reason car engines look attractive, but i think you will find that if you drove your subaru at 135 hp all day it would not last like you think it might, and it would burn way more than 2 gallons an hour (30 mpg at 60 mph?). because at 60 mph it is not using 135 hp or anything near it.
what would be interesting is if you drove your subaru on an empty highway at 110 knots for a few hours, how much fuel would it burn then? (at cessna speed)
that asked, it probably has a better specific fuel consumption than a lycoming.
diesels have better specific fuel consumption than air cooled gasoline engines, they are expensive, and unless you fly a whole lot, you will never save enough in fuel, to pay for the overhaul difference, let alone the purchase price premium that currently exists.
i think in general , converted car engines do not go the distance , when put in planes... there may be exceptions....
Fuel burn in auto conversions is comparable to fuel burn in an aircraft engine designed with electronic ignition and FI. BSFC is a bit better than the old Lyc and Contis, but not orders of magnitude better. A 165hp Subary conversion running a typical 65-75% cruise will burn maybe 10-20% less than an equivalent powered lycoming with fixed timing and a carb. Having P-mags with variable timing and running lean of peak with tuned injectors will close up that gap to almost nothing though.
As was mentioned, piston speed IS a good indicator of engine longevity when discussing piston wear. And EJ25 making 160hp at 5600 rpm and 79mm stroke has about 60% higher piston speed than a Lycoming O-320 making 160hp at 2700rpm with a 98mm stroke.