So a few days ago I decided to get my exhaust redone, I removed 2 out of the 4 cats on my 4.0L. I removed a cat on either side of the headers, right after the sensors, that way the engine/computer would not know the difference. So my new set up is 2 stock cats piped into a single pipe using a custom made Y pipe, then its piped into a flow master single in/out 40 series muffler, and dumped before the axel. All the piping was custom welded and bent to have as little creases and bends as possible, to maximize air flow.
Seeing there are 2/4 cats, i still get enough back pressure to not lose noticeable low end power/torque seeing on a NA engine, some back pressure is good. But where i really noticed the difference was high end RPM, where the increase air flow helps. It has this quickness and agility about it at higher speeds now, and i love it. so the combination of back pressure and air flow has been great so far.
MPG wise, i will give an update when i have time to go through a tank of gas on regular drives, but for now im enjoying my exhaust and have been getting on the throttle a lot!! Also i just installed a BakFlip so i will see how the combination goes.
Sound wise, i love it. I've had LOUD vehicles, from a 89 blazer with a 350 block engine, no cats, and a flowmaster, to a WRX tuned for 300+ WHP, straight piped from the turbo, no cats or muffler... so i have had LOUD and OBNOXIOUS vehicles... but this combination sounds awesome. Its loud when you hit certain throttle percentages, but when cruising it mellows out. During idle it sounds nice (ALSO lowered my idle rpms), when WOT, its got this turbine sound to it and throaty sound. Sounds amazing, and my dad who has a magnaflow on a hemi ram, said that for a V6, it sounds beastly.
So overall, i am super happy with the results of my exhaust modifications.