I was wondering where to get some good high flow cats for my 99 F-150 4x4 with a 4.6 L V-8. Any help would be appreciated. Also I want some of your takes on cat back systems. I have a local shop that will put on dual 2.5" for about 300 bucks with tips no muffler. I want to know if stepping up to 2 pipes out of a single y does anything for flow. The stock configuration brings the drivers side and passenger side together at a Y then goes back as one pipe. I personally can't see how going up to 2 pipes from a Y increases much unless your original 2.5 pipe isn't sufficient to flow the cfm the engine is putting out. I can see how if you split the pipes where they Y into say an H configuration or simply run duals from the Y point how you will increase cfm (flow). Someone tell me if my thought process is wrong here.

Thanks in advance for the posts.