Is there any point in going with a 5" cat back on an otherwise unmodded 2006 (ie lower EGTs) or would I be hurting performance vs. just replacing with another 4"?

 

if that is all you want to do then go with a muffler delete pipe and a nice stainless tip and save the money

 

Until you start moving a lot of air, like with twins or high revs, a 5" exhaust won't help, and may hurt the performance... the 4" exhaust helps to keep flow velocity higher, which allows a higher pressure delta across the turbine.

 

Thanks for the quick replies. Anyone have experience on the sound difference between an MBRP muffler and a muffler delete?

 

I don't like either... I prefer AeroTurbines

 

Any way to describe, in words, the difference? I haven't heard one.

 

A muffler like a MBRP which has packing will kill any turbo sound out of the exhaust. Also, when they get sooted up they get loud and raspy. I speak from experience.

A baffled or louvered muffler like the Aeroturbine (or my Donaldson) lets you hear the turbine whistle out the exhaust. It's subtle with the stock turbo, but it's there and it's sweet. Much more noticeable on a truck with a bigger exhaust housing. Additionally, the mufflers sound deeper, and they don't get louder with age. They don't have the "smooth" sound of an MBRP when new, but they do sound good.

If you want super-deep, go with a 5" diameter muffler in your existing 4" system. That's what I have, and it is deep indeed. The bigger muffler core allows the sound waves to develop...

 

The place I bought my exhaust from is ofb but the muffler is the same as the donaldson (louvered inside) and I get compliments from ppl saying my truck sounds wicked. I have a few miles on it and so far so good, not sooting up and changing tone. Most likely my muff. is a copy but it's 5" through.
I don't know if going stainless really matters to you, I went with aluminized-cheaper and does the same job. Where I live, I don't have to worry about it rusting up. I think going stainless is for those who have to worry about rust and for those who have a little more $$$. (no offence) I only paid $335 to my door for the whole system-5" turbo back.