Anything from the Cat back will generally not void your engines warranty.

we have seen three late model dodge trucks lately that the owners put "high
flow" cat-back systems on and immediately started having issues.  most
usuall sympton was rough running and loss of power.  all three times our
wrench took a big pair of channel locks and squeezed the tailpipe(s) almost
closed and told them to take it around the block.

on every occassion they came back and said it was much better.  we then told
them to go back to the muffler shop and have their expensive high flow stuff
cut off and put the stock systems on.  a couple of them got really upset at
us and said we needed to find another way to fix their problem cause they
had spent a lot of money on thier new exhausts.  some of the systems work
and some don't.

the moco pays thier engineers a lot of money to design all of the components
to work together.  why pay lots of money to undo all of that?  unless you
are going to make other significant changes to the engine don't mess with
the exhaust.  just my 2 cents.
michael