I don't really know, but as LFS doesn't propose to change parts, I think it is not usefull to propose "performance air filters" or "turbo stage 2".
I think it is out of question to change parts. But it is important to be able to change parameters.
For me, LFS is not giving to us real cars that we can set, but "prototypes" that are being set, like in real companies, where if something doesn't work well, you do it antoher way.
So i think things that should be set would be length of the tubes (inlet and exhaust) where the only thing which is done is to change the torque curve as a function of your track.
Then, as it has been told, it would be great to change things such as ignition timing for exemple, and injection time. All that would give more or less power. Keep in mind that more power implies more consumption. So maybe some pilotes will prefer a light car with low fuel but less power, or others more power but more fuel onboard. With that, you can adjust the balancing of the car with adding fuel in the tank and compensating by having more power.
You should be able of choose the diameter of ports. Bigger will help at high revs, wether smaller will help at low revs.
Also you can add filtering systems that are more or less efficient: the more the air is clean, the more your engine is reliable, but you have less power.
You should be able to change the inertia of flying wheel, which influences acceleration.
You should be able to change things that are automatically balanced by a disadvantage. So you have homogenous performance, but optained in a different way.