MVS Forums Moderator JDS60R tells it like it is on high-flow catalytic converters (“high-flow cat”):
Put in a Magnaflow. They have both a federal (49 state) and a CA mas well as universal and Oem style ( tougher wash on it so less trouble for vehicles that throw codes easily. The center resonator has so much more restriction that an open pipe with stock resonator doesn’t pick up too much power. Weld in a Magnaflow high flow cat and a magnaflow center resonator (muffler) and you will see the power increase as well as lower exhaust valve temp, spool up time, and raise intercooler efficiency as the turbo is no longer fighting the back pressure.
This is the only solution I have found that helps the motor, makes power and keeps me code free and legal. I started doing this 5 years ago (always replace O2 sensors if more than 100K miles old) and no codes or failed emissions to date. It runs exceptionally clean on the emissions test. Also remember to put the stamped number of the Cat facing down as required by law.
I prefer to tig it in with 309L but you can mig it if you want. On some applications (3 inch and bigger) I have to weld in a center bung on a magnaflow 46009 or 446009. It voids warranty but we never have an issue with them and they are $100 or less. 2.5 and smaller have mid bed O2 bungs available.