oh yea i have a 2004 wrx too. forgot to mention that
i am having the same idle issue that ed05wrx is having after doing this. driving around is fine but if i am just idling it starts to idle like it wants to stall out and the lights will dime in and out as the idle slows/goes way down.
i have it set up like diagram A right now with the plug closest to the sensor going under the intake mani and the plug furthest away from the sensor going up to the nipple pointing down.
but before i did this it was set up opposite like diagram B. the plug closest to the sensor went up to the nipple pointing down and the one further away going under the intake mani
I don't know if this could be the reason for the rough idle or maybe my battery is on its way out but you seem to know way more about this than i do so any help would be great! but either way my check engine light did turn off! which i am greatful for.