maybe all of your could help. i have 2017 forester XT. last week i swapped out the exhaust to a wrx stock midpipe and then put on nameless 5 inch mufflers.
the car seems to have less torque now, the MPG has gone from averaging 17mpg to 19mpg. the the car wont let itself go in to any boost pressure over 11psig. i have been testing in I and S. S# made it better. but thats not the solution im looking for. what is weird in that the car is getting MPG numbers more like what the factory advertised, yet the entire ECU is acting like its burning to lean.
so my car did the opposite of what the original posters did. stock wrx midpipe with nameless 5inch mufflers.
ill be going back to stock, im going to ask subaru to look for any weird codes. either its the exhaust or my A\F ratio sensors decided to go wonky at the same time.