The 6.1 actually uses a 2.75" system from the factory (2.25" on the 5.7), but it does have some restrictions in the pipes that are smaller than that. Long tube headers and bigger pipes generally free up a few extra HP.
Bigger pipes have a habit of droning badly on the 5.7 when the MDS kicks in and it drops back to 4 cylinders, not a problem if you disable the MDS though.
I've just ordered a cheap and nasty set of OBX headers for the LX platform, playing with a slightly different swap.