Some other things that rid me of misfires an idle/stumbles: Fuel injector seals & intake manifold gaskets. They just sort of shrivel up and decay at higher mileages from heat cycles.
It seems subaru moved to a cheaper fuel injector seal that only seals against 1 surface, vs the originals that sealed against 2 on the injector tip/face. This required slightly crushing the fuel rail to get a positive seal against vacuum
While there i added some grimspeed 8mm phenolic spacers, removed the coolant feed to the throttle body and completely removed the EGR system.
Then the NGK BKR5EIX-11's seemed to do better than any other plug I stuffed into the car. Last forever and worth the price.