EXHAUSTed....still

It sounds like you're thinking all Y pipes are created equal and that is absolutely not the case, and any high end Y pipe made for these will not hinder anything at all instead they will help everything performance wise, unless you are some awesome big hp forced air build but thats a whole different deal.
The factory Y pipes yes you are correct they are terrible because they are crushed Y's with way to small piping but it's to please all the regulations. Pictured is a example of my current Y pipe, made by Kooks with there high flow cats next to the factory Y and cats, you can see there is a huge difference. I have Kooks 1 3/4" long tube headers and I've played with all kinds of setups and tunes for each setup, including running the open header which sounded awesome haha but was a turd on power because way to open. Also pictured is my DEQ pipe setup that I made so I can get through emissions without having to remove the long tubes, no Y just open pipes with factory cats and dumped thats it, also a turd but not as bad as open header. Slap my Kooks Y on and bam power is amazing. As mentioned I've had custome tunes made for all these setups to see if I could get them to run better but just no comparison to having a good Y pipe. These 5.7's have no chance of sucking in more air than they can push out of a quality made Y unless they have some pretty decent sized forced air.
Also I wanted to point out that the muffler alone isnt just what decides loudness and in cab volume as you stated, I could make a very loud exhaust setup with many different mufflers and still have it pretty darn quite in cab, changing location of exhaust exit/exits, tip size and shape, where the muffler is located, resonators, matting, piping size, header/manifolds used etc etc all play into decibel levels, tones and in cab sound levels.
Also OP stated they live in Cali and as most people know they have pretty much the most strict emissions testing of any state so theres a lot they cant do or have to consider.
Just sharing some quality info
Picture 1" my DEQ pipe setup.
#2: KOOKS Y with there HF cats vs factory.
#3: you can really see how bad the crush is on the factory Y in this pic.
#4: just for fun threw in a KOOKS long tube comparison vs factory manifolds lol

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I had my muffler shop get rid of the crushed section of the y-pipe and just ran true duals from just before the y and down to an x-pipe.

 

How much did that run you? All I've got going right now is a muffler delete, I left the resonators in place.

slbenz600 - You also have to consider that RamthisGirl has a 5.9L Cummins vs a 5.7L Hemi and is in California; different rules than Utah.

 

I want to say it was about $100 with what all I had done. I was running a single 1-chamber Flowmaster and had them cut out that crimped section of the y-pipe and just turn it into 2 pipes, then run those into an X-pipe I brought and run the pipe down to another 1-chamber Flowmaster I brought.

Why would a Cummins need a y-pipe? They have a single downpipe.

 

Whoops lol I assumed it was a hemi truck as well
My apologies haha that changes some things

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You have to ask RamthisGirl that question. I thought they were all just a single pipe into a trashcan muffler and out the back. Then again, I've never owned a diesel truck so my knowledge is very limited.

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I've owned one and worked on thousands. That's why it kinda got my attention.

Seems as though hers is an 02 5.9 360 engine, not the 5.9 Cummins.

 

Huh, seems I missed that it was an 02, I got it in my head it was an 04 for some reason. Thanks for the correction CorDog009.

 

I had to go search under her posts and found it there, it wasn't in this thread.