These low SAPS oils have very different additive chemistry than we're used to, and the few UOAs I've seen in gasoline cars with them seem to indicate they have much better TBN retention than traditional chemistry. VW gasoline cars have a recommended OCI of 10,000 mi on 504/507 oil, though VW engines tend to have large sumps for their size, so you might not be able to go that far in a WRX. I have zero experience with VW 505 oils in a gasoline engine though.
The Lubro-Moly additive is a good choice for break-in, but likely not continued use. The LM additive is primarily MoS2, which is an extreme pressure anti-wear additive, a very different beast from soluble Moly-DTC found in most motor oil formulations which at levels over 75ppm is primarily a friction modifier, not an AW additive. The Moly in the factory fill is a combination of soluble Moly from the oil and MoS2 solid film lube wearing off the piston skirts, a UOA can't tell the difference. I suspect Subaru want's to keep that MoS2 in the engine until break-in is largely complete, and it's thus a good idea to use the Lubro-moly if you change the factory fill early.
I changed the factory fill early on my WRX, then used the Lubro-Moly until 10k mi, see my UOAs linked in post #2 for the results. I now see this was unnecessary. My 2011 Civic kept the factory fill for 6,000 miles, and wear actually went UP in the second UOA. Those UOAs are posted at BITOG under the same username. My mother just bought a 2013 Legacy 2.5i, we're going to leave the FF in that for the full 7,500, we'll see how the UOA comes back.
All that is a long winded way of saying I don't think the wear metal levels in a factory fill are anything to worry about. They're always lower than the condemnation limits set by HD diesel engine builders for long term OCIs. I mention them because they're the only ones to publish such limits.