My Evo is only boosting 12psi and the only thing i did is 3' AMS Turbo Back Exhaust and a K&N filter adapter

 

Check to make sure your boost solenoid is still plugged in under your intake. I'm not sure if it is a normally open solenoid but if it is it would do that. Good luck

 

11psi is wastegate. And it's not uncommon for a boost guage to read 1-2 psi off. I'd guess something is lose.

Check all your boost hose fittings.

Also, since you've got an VIII model, you can reset your ECU in an effort to help it 'learn' the new bolt-on and actually gain some hp. Do a search on it if ya like.

 

i would say you have a vaccum line off of your solenoid or you left the solenoid or maf unplugged while you were messing with the filter.