Now that I have fixed my boost problems, I just thought I'd close out this thread, in case anyone else runs into the same problem. Per above suggestions, I replaced MAF, BCS and the few remaining vacuum hoses. After doing this, my boost increased only very slightly to about 9 PSI. After thinking I was going crazy, I realized that I made a rookie mistake and accidentally threw out the restrictor pill when I replaced the last few vacuum hoses. After getting a new pill from Subaru for a whole $2, my car is now making 17-18 PSI boost. Thanks everyone for the help.
As an aside, my target boost is actually 16.8 PSI, so I am now slightly overboosting. I will try get hold of my tuner for advice (XPTTuning/Braden), but if I have no luck, I'll post a separate forum thread about that.