Arouse the DAMPFHAMMER!
In my former job we made harness holding up to 150 °C. We used for that a not ROHS conform wire material, you need an exception allowance to bring such stuff into the market, as most insulations are just capable of 125 °C. The M3 spec of 80 °C is not too high, but I believe it won't melt immediately if it is 90 °C. The temperature alone of the engine bay isn't enough, there has to be a sufficient heat transfer to the harness to warm it also up to 80 °C. So if you check for heat transfer, like touching surfaces (connected to oil = 90+ °C, to coolant = 80+ °C), then it might be good to hold them above of it via a cable bracket/clamp.I guess I’m not sure what the ambient temp gets to under the hood, if the coolant and air /fuel are all sub 200 F then the question would be how much does the header make it rise , and if so what temp drops could be seen using exhaust heat wrap.