Nissan simply could’ve never acknowledged it. Instead made a point that the GT-R was untuneable, which to my knowledge, is the only time a car company has claimed such a thing.
Heat exchangers don’t modify emissions performance. It’s ECU tuning that they’re worried about.
Toyota has actually gone to extremes to lock down the Supra ECU to prevent tuning. Last I checked, only one company in the world had figured out how to unlock the ECU and you have to ship your ECU around the world to have them unlock it.
This seems more likely to be warranty-related since it's very easy to break mechanical parts by adjusting things like boost pressure and ignition timing.
Modifying modern cars for high performance impacts the emissions systems. In many cases the ECU modifications will actively defeat certain emission control measures to allow the higher power output without check engine lights, for example.
Showing a good faith effort that you've designed the emission compliance systems to be reliable and hard to defeat is part of the process.
There's a similar thing that happens with WiFi access points: You have to show that you've made an effort to prevent the end user from trivially modifying it for higher power output or other changes that would deviate from the way it was compliance tested.