I did dynos after installing the headers, and I actually LOST 10rwhp, same dyno and same tuner. The tuner said he had seen several other cars (350Z) that had the same result, and wasn't sure but thought maybe it was the cats.

So a couple of months later I swapped out the stock cats for some high flow cats and did another dyno, and bingo...an 18rwhp gain. No other changes in the interim. So one conclusion might be that there was a bottleneck caused by the cats and the headers may have opened up the flow from the exhaust manifold, but something was choking up or negatively impacting the flow. THe new cats freed something up. But to get a net 8hp with headers and high flow cats.....not sure it is worth the money and the installation hassle.

A cat back is probably worthwhile and it helps with weight potentially as well, depending on which one you select.

 

How do you know that the high flow cats alone wouldn't net you higher horsepower?

 

I don't. And I did not say that high flow cats alone would not improve hp. Indeed, given the experience, my assumption is that they should increase your hp if added by themselves. However, it is only an assumption because that is not the order in which the modifications were made. They went 1) catback, 2)header, 3)high flow cats in that order.
Why? Because this is an '03 and the mods were coming out in that order early in the product phase. Today, two years later, the number of offerings by category, and in each category, is considerably better than back then, as are the quality of some of the items.

What is important to recognize is that any single modification may garner a fine improvement. But the aggregate gain of making all three is likely to be less than what you might total up if you just did one header mod dyno, one high flow cat mod dyno, and one catback exhaust mod dyno. There are diminishing returns. Thus, given the costs and labor and hp returns, my inclination is to not do the headers, and to do the others. Now, if you do not want to mess with the cats, then doing the catback alone would seem to be the path to follow, if my experience was any guide, because to spend the dough and get a drop in hp would be a bummer. Since the catback seems to be the big hp gainer on the exhaust side, if you were therefore only going to make one change, that would seem to be the one to make.

Hope that helps.