It's all about Combo Matching.
If you just plan on keeping the stock cats and get a good deal
on a 2.5" cat back, then I'd say you are going to be fine. But
if you are building a complete system and delete the cats or
get hi-flows, I'd use 2.25" unless you are planning on a stroker
and then I'd consider the 2.5" system. I can tell you exactly
what will be the best arrangement for you if you share your
entire mod plans with us, it is pretty well established what
works on a V6 now. If you are staying close to stock and
just want a local shop to customize a set of true duals
then you could stay with 2" pipe all the way through.
Gets confusing doesn't it?
That is why I need to know
what your plans are, so you can do it right the first time.
That makes it cheaper for you in the long run.