When I bought the truck, it has just some bullets and dumps and it was LOUD! So, to quiet it down (not worrying too much about performace at the time wanting to make it more streetable), I ordered some 3" Summit Turbo mufflers because they were cheap and would do the trick (quiet) until I decide on exhaust.

How much do you think they are hurting me? I have 3" pipe from the headers to the mufflers to turn downs. It's pretty quiet.

They are quiet and cheap,but way to restrictive.I had a Summit split rear system on my 98 Ram and just the other day had to replace the muffler(turbo).I bought one of their "race" mufflers to replace it (flowmaster copy).I am already seeing pretty good gains in fuel milage.Based on that,the turbo must have been a big kink in the system.If you look inside it,you can see why.
Oh,but it is way louder!

IIRC, the Summit turbo's are not 3" all of the way through...I think they are 2.5" inside. At least that was the case a few years ago.

I replaced my 2.5" summit turbos with 3" bullets and saw no gains. But my motor only makes probably 350-375 hp.