I just got a set of pacsetter lt's for my 05 5.3 sierra Will I have enough backpressure, scavenging, and velocity with dual 2 1/4" pipe with an x-pipe with no cats and mufflers? I have 2 1/4" piping right now with but could go up to 2 1/2 or 2 3/4?

 

you should, back pressure isnt necessary and our motors run well even with little pressure, you should be alright without the different piping, you can always tune for it and the worst that would happen is lose a little down low IMO

 

Is my mileage going to go down or stay the same?

 

When I added a dual 2 1/2" system to my 5.3 the mileage went -10%.
When I added a Whipple charger to it, it went +20%.
My suggestion is to stay with a 2 1/4" system if you're going N/A.
.02