High Flow Catalytic Converter Problem?

[JIBBBY]

I INSTALLED A MAGNAFLOW CATALYTIC CONVERTER IN MY MODIFIED 1992 SC400, MY QUESTION IS WHY DID THE CAR LOSE LOW END AND GET TOP END, AND ALSO THE MOTOR CHOKED OFF THE LINE. I HAVE 2 1/2 EXAUST PIPING FROM THE CAT TO THE MUFFLERS AND AM RUNNING BORLA'S. I REMOVED THE CAT AND RE-INSTALLED MY OLD CLOGGED CAT AND GOT MY LOWEND BACK. I WAS CONSIDERING INSTALLING HEADERS BUT I WONDER WHAT THE EFFECTS THAT WILL HAVE? CAN ANYONE ENLIGHTEN ME DOES THESE 4.0L MOTORS NEED A CERTAIN AMOUNT OF BACK PRESSURE TO RUN SMOOTHE?