[PabloX]

A lot of companies make build vs. buy decisions for what may seem like bizarre reasons from the outside. Sometimes it's basically a matter of time to market. Their own engineering staff may be too busy with other projects, for example.

Porsche has also done plenty of work for other big auto manufacturers. They designed the Ford Duratec V6 (the 2.5L used in the Mondeo/Contour, Jag X type and others). Clearly Ford has the internal engineering resources to have done it themselves but didn't for some reason. Also, it's well known the Porsche designed the Harley V-Rod engine and they also built the Audi S2 and cars for Mercedes. Similarly, Yamaha designs and builds engines for Toyota in certain cases (like the 1.8L used in the Lotus Elise).

You mention a comparison to the Boxster engine but if Porsche did indeed design part of the Subaru engine, they most certainly designed it to Subaru's specifications which are different than what Porsche wanted for their own cars. I also thought there's a firing order difference between Porsche and Subaru flat 6 engines, but I could be wrong on that.