infidel

Anyone who tells you they aren't louder is pulling your leg.
Without a muffler it's not much louder in the cab but to folks on the outside it's a lot louder.

shortround

I've had dual 4" on mine for about 6 weeks now.
Seat of the pants meter indicates that there is a definate power increase, especially above 1800-2000 rpm. My opinion is that with the reduced backpressure, the turbo spools up much faster, therefore bring the power on faster. It's quieter than stock, running ~80db at idle and ~95-97 at 2000 rpm. The sound level measurement were taken ~25 ft behind the truck. Interior sound is greatly reduced.
I did see a fuel mileage increase. 8.5-9% in combo driving and 10-12% in highway, depending on speed.
The truck just seems to run better. Maybe it really does or maybe it's because I was telling myself it would.
I don't think you will gain anything with duals as compared to a good single system, since the single 4" flows very well as it is. HOWEVER, I do like being able to listen to the exhaust on my side and it looks good.
There are several manufacturers out there. Of those that I looked at, the quality is good. Ease of installation will vary a bit. The price will vary a bit. It's all in what you want.
In short, anything you do to improve the exhaust flow will help.
OBTW, if you are looking at a muffler with a single inlet and dual outlets, BE CAREFUL. Many of those mufflers are very restrictive. I first ran a 3" inlet, dual 2.5" outlets. It would appear that the flow would be better. NOT. Internal construction was such that the 7.1 sq inch area of the 3" pipe was actually reduced to ~ 6 sq inches. It looked good and sounded good for everyday driving. However, when rpm's got above 2000 or when pulling a load, it started choking down. Enough heat was being retained that I had discoloration of the exhaust pipe just in front of the muffler. That tells me there was a restriction.

PumpDaddy

The 4" is much louder than stock, especially around 1500-1800 rpm with a load. At idle, it's not anymore louder than stock, but punch it , it starts out normal, then goes to a loud, SWEET roar, it seems to settle down at higher rpm's, and anyone within ear shot will hear it roar and turn to see where it's coming from. The police haven't bothered me too bad, I just blame all the smoke on bad fuel and tell them the pump is loading up . The last county sheriff that stopped me, even looked up under the truck, had a few choice words as too how big the pipe looked, but mostly grins, AFTER he admitted to owning a PS. His last words where "injector pump my butt, get outta here before I get really po'd"

KATOOM

Speaking from experience, I feel taking off the muffler BUT leaving the resonator on is a great move. You get almost no gain in interior noise but outside sounds great. You do get the straight through gain since the resonator is already straight through and 4" to boot. Plus you dont get any annoying resonating around the 2000-2200 mark. The only reason I went straight piped is because the shop who took off the mufffler did a terrible job and I couldnt stand knowing that so I had someone redo it later and just get rid of the resonator while they where at it. I love the sound now but I hate the resonating while towing. It is quite loud though. You have to like that cause you will get some looks.

BigBlue

I've got 4" straight pipe dumped under the bed. It's loud inside and out. It resonates at all speeds. But I love it. You know when I'm coming.