Exhaust upgrade - Go for header first?

[DuratecNC]

It’s the most effective part to replace by a long way as far as power and response is concerned. Slightly more noise, nothing bad at all.
But then read here:

Exhausts, Catalytic Converters and Emissions

Added 5/2/21). In addition, various models over the years have different EURO EMISSION RATINGS. Therefore, each rating had different emission values to the previous one. So it is important to check which rating you have. Clearly the ND’s will be the latest with the most current stringent values applied. Just my opinion that if I had the latest spec ND I probably wouldn’t bother swapping modifying as it’s not really worth the bother. I know not whether they will pass a current emissions test wi…