Well, I use to have the complete LCE exhaust.
Then I switched to the stock manifold and downpipe and kept the cat-back from LCE.
I noticed a slight improvement in lower end performance when I did this (that's where I run ALL the time).
My suggestion to you is have someone weld up a custom exhaust (2" cat to muffler, then 2.25" muffler out to the back) and keep the manifold and downpipe the same.
I think you'll notice an improvement in your daily driving RPM ranges more than just doing a header.
When I did the full LCE exhaust, I noticed an improvement on the highway, but nothing in town.
Just IMO,
Todd