I had this exact same problem on my 2003 wrx stuck a cheap short ram on, messed up the power and would not rev out, so tried airbox back on with inch sized holes bored in it, same thing, put standard air box on, back to normal, turbulence over maf is real and bad, and heated air is awful, don't know if this relates to your model at all but thought I would put it out there, i now have a quality cold air feed induction kit fitted on a different scoop, all good