Here's some hypothetical exhaust mod plans...
(Disclaimer: These hypothetical modifications are for hypothetical OFF ROAD USE ONLY. Federal Regulations prohibit actually disabling emmissions control equipent of street-driven vehicles. Cheers!)
I've edited the pic posted by AutisMDXTypeS to show my hypothetical modification plans. I'll hypothetically monitor mpg's between stages.
Stage 1: Hypothetically gut the 3rd cat. I think it's only there to get the "Ultra" part of ULEV. Without it, it hypothetically meets the regular emmissions requirement. It then oughtta flow a little better, hypothetically. MPG test hypothetically in progress.
Stage 2: Hypothetically remove the factory 3rd cat and feed a replacement splitting-muffler directly, Magnaflow 1in-2out (3"in-2.5"out). This will give better flow compared to the kludge of a splitter after the factory muffler. The splitting muffler's outs will transition to 2" flexible
stainless tubing and stepped down to the 1-5/8"OD pipes leading to the factory mufflers.
Stage 3: Replace rear mufflers with magnaflow 2" units. More flow, more noise, might be too loud for the wife? (I know my f350 with just those two mufflers is good-n-loud in WOT).
Stage 4: Install 3" magnaflow cat at location of factory 3rd cat, install two O2 sensor fittings after it and move the two after-manifold-cat sensors to that location. Then hypothetically gut the manifold-cats. Emmissions are hypothetically fine as long as there's an adequate cat in the 3rd location. A correctly-selected magnaflow cat would do the job.
Or, skip 3 and just hypothetically do 4, get a quieter ride and maximum power/efficiency for that quiet-tude.
Here's the original pic...