I just tried this filter and I must say it seems like a poor fit for the WRX.
Let me first say that I am a huge fan of Baldwin filters. I work at Centerline Alfa Romeo, a major parts retailer, and we only sell Baldwin filters for both 4 and 6 cylinder Alfas, literally thousand a year go out to our customers. I also have personally used Baldwins on my Alfas for 10+ years. So, no question, I think they are a great quality filter.
The problem with the B161-S is fit. As Santofontana referred to, this filter doesn't engage very many threads on the fiter boss. In my case, I got about 2 turns of the filter before the gasket contacted the mounting face. It didn't feel right to me, so I grabbed a tech from the independent Toyota repair shop next door, and he felt it and concurred that there was no way he would install a filter that fit like that, on either his car or a customer's car. I tried a second B161-S - same result.
I grabbed a factory Subaru filter, which gave me 6-7 full rotations of the filter before contacting the mounting face. Much better.
Maybe the Baldwin B301 would be a better fit, as it's listed by them as a direct cross reference to the factory/Purolator filter.