I agree the stock system isn't too bad and a great way to go without upsetting the applecart too much. Also agree about our 03-08 Hemi engines keeping up and sometimes passing the newer 5.7s. They're great engines but with open exhaust and superchips tuner on mine I win every time against my friends 2013, especially down low.

See personally I'd disagree about the theory of back pressure being needed from my own experience and also what I've seen on countless muffler A/B tests and non muffler dyno testing, on top of that the recent Engine Masters dyno testing Muffler vs No Muffler vs high performance mufflers pretty much disproved the theory of back pressure being needed for max power and power under the curve. No muffler won everywhere in the powerband on their testing of a Big Block chevy. I will say that often times muffler may feel better because its reducing noise and vibrations and creates the illusion your engine is running smoother/more power because you can't hear it as much and can concentrate on the way the vehicle runs instead of sounds.

What matters to performance much more than mufflers and piping is the manifold & header design within 4 feet from the cylinder head, that sets the tone for the rest of the system and anything past that point that is wide open and non restrictive will benefit performance. You also then have the theory of X pipes which basically help equal the pulse from the exhaust to scavenge the system in a true dual setup prior to mufflers, not needed in a open exhaust scenario. Top Fuel Dragsters with zoomy headers would also disagree with back pressure being best for power.

Watch the whole episode of this after you see the spot at 11:09 where it talks about 2.5 inch to 3 inch mufflers regarding back pressure.


On mine I'm running JBA mid headers with stock mid pipes (modded to fit headers) running stock cats on both sides and stock Y pipe and it just screams and puts on a show. Basically I didn't intend on it to be exactly this way, my new muffler blew out in a short period after I bought the truck from my friend and the resonator was in good shape but was hanging off the truck and muffler had disconnected itself from y pipe while I had been snow drifting and pounding my truck. Took all that stuff off and got a wet back in the snow storm and loved the way it sounded and kept it.

 

The 07 has a resonator in front of the muffler just behind the Y behind the cats, then a large muffler and another resonator in the tailpipe. The pipe size from the Y back is 3" on the Hemi. It flows ok, but has enough back pressure to give good low end torque. I have a Magnaflow cat-back that flows much better, but I lose a bit of low end torque, but gain power in the higher rpms.
There is either a back pressure issue somewhere or you have gotten a bad batch of mufflers. The Hemi is a good engine, but it doesn't flow enough air to just blow up mufflers.
It sounds like you have underlying issues causing the low mpg, lack of power. It could be misfiring and or getting unburned fuel into the exhaust and it could be firing off in the exhaust system causing the muffler failures.
I would have a shop drive it with a proper scan tool and watch all of the perimeters to see what isn't doing what it should be. The 07/08 engines are the same, so you may have a sensor issue or a wiring problem from the engine swap. The only difference in the 07/08 is that the 08 did not have the front resonator. Just a muffler and a resonator in the tailpipe. That has nothing to do with your issue.