Yes, the stock cat is very restrictive. It doesn't matter if you can see through it, the flow rate is horrible. If you need to keep a cat for emissions reasons, get an aftermarket one otherwise ditch it all together.

 

I haven't done headers myself but would like to. A friend did headers and a Ford Motorsport cam and was very happy with the result. The cam has been discontinued , but apparently still available through Crane as they made it for Ford. So I've been told. As for the cat I picked up about 1.5 mpg and noticeable power increase by getting rid of the old cat and adding a free flowing exhaust. I did dual flow masters , but see a lot of merit in those that prefer a good two into one 3 inch system. It's probably best to go with a high flow aftermarket cat while doing the project rather than leaving it out, but that just depends on how you use it and what you want to do. Im curious to see what others say about header choice. That may be in my future.