The headers I bought claim no CEL will be displayed and make between 30 - 40whp across the board and actually increase midrange torque. They also give an option for a cobb tune but only marginal gains are to be had on top of the headers.
I looked into it a lot and the company seem very well regarded in Porsche circles so will give them a try. Very interested to hear how it sounds. Should be epic.
Here's a link to the headers:
https://www.dundonmotorsports.com/porsche/981-gt4-spyder-race-headers-425hp?carid=41
And claimed gains after dyno back to back with a standard car on the same day:
Take American BHP claims with a pinch of salt, I had a softronic map on my car, which made claims of circa 25HP and had dyno graphs to prove, when I did before and after graphs at Powerstation it was worth like 4-5HP tops.
But yes these cars to gain from headers/exhaust, I did X51 headers on my 911, those netted around 8HP and more power everywhere, I then went de-cats with an X-pipe which netted 20HP gains above 5500rpm, but lost huge torque, so switched to some HJS 200 cel cats, which still triggered the dash light, but car sounded better, it gained everywhere, but in the end I think my car went from 356HP stock to about 368-372HP with cats, softronic, X51 headers, it was to put it bluntly a waste of money, yes de-catted it pulled 390HP and it could be really felt on road but so could the 50lb/ft loss down low it felt so flat.
Those headers look nice, just be warned, Porsche are very anal over any kind of modification, on a new car warranty you can get away with it to some extent as they have to prove modification caused the issue, but once you move to extended warranty they can just deny putting warranty on the car or a claim due to a modification like that.
You gonna get a dyno done before and after, shall be really interesting to see what you make, because you essentially have the updated 3.8 C2S engine I had in your car, but of course the version that won't melt itself.
Moving to the GT4 from the M3 must be a game changer, one is a tail waggler that struggles for traction and the GT4 is like a planted go-kart in comparison.