Well, my '07 has a cat on each side after the manifold then a 3rd larger cat @ the Y-pipe union. I went with the Dynomax 2-1/2" cat-back kit to get a better sound and flow. I noticed a little top end improvement and better sound but no other real changes. On mine, the Y-pipe has some significant factory kinks in it for clearance that appears to be more of a potential flow problem than the factory manifolds on the 4.0 SOHC.
For less than $200 delivered, the cat-back system acheived what I was looking for. I have doubts regarding the cost of headers-vs-effectiveness without addressing the Y-pipe & the factory cats. When you add those to the equation that is a lot of $$$ for what the potential gain would be on a stock engine. That's my opinion.