I recently upgraded to headers. Because I am from California, installing Cats is a must due to the strict emission laws. However, even with the cats installed, I was getting the check engine light. After doing some research, I conducted the defouler trick.
After peforming this fix, my car has logged over 200 miles and my check engine light hasn't come back once. I scanned my car for hidden error codes with my Peake Reader and it seems like the "inefficiency" codes is a thing of the past.
I have a better picture of the modified/drilled out defouler if anyone wants to see it. The hardest part is drilling out the darn suckers. It is tuff and you get metal shaving/shapnels in your eyes, ears, balls, and mouth.
Good luck with this DIY. Again, your mileage may vary depending on your car's setup.