Well, if you don't connect the y-pipe and drive it with open headers exiting just in front of the driver and passenger floor, then it sounds beautiful - especially at wide open throttle. I have a stock cam and engine and I couldn't believe how much my bone-stock Tahoe engine screamed with open headers. I swapped a T56 into my Tahoe a few months ago and going through the gears with open headers is so loud it's almost violent, but it's violent in a good way that makes Prius drivers pee themselves when you blow by them. On my way to the exhaust shop to get the y-pipe welded I passed a Prius and I couldn't stop laughing.
As far as how it sounds after connecting the modified y-pipe to my stock exhaust system, it is barely noticeable while being really quiet and stealthy. I was aiming for a stealth sound as much as possible so I like it. It has a nice presence when it's idling and running, but it's quiet enough that I can drop the hammer and not catch the attention of the local law enforcement.
Even better than the stealth sound is the increase in power. It's not like a supercharger or a shot of nitrous, but I have lots of extra power in the low and mid RPM range. Increasing the low and mid power is the biggest reason I went with 4-2-1 headers. Unfortunately, I couldn't tell you about any improvement in mileage because I can't keep my foot out of the throttle. It's loads more fun to drive fast and hard.
Once I get the lead-foot out of my system I'll post MPG updates.
