My thought on this is:
By tuning your car ,isnt UPGRADING it but customizing it
What i mean is that you have cams timing,A/F mixture and this stuff
Each will have a unique effect not only on the cars performance but damage,like engine is heating fastest when you run very high rpms or too much boost,you can have broken pistons and white smoke coming,broken transmission because of a bad downshift or bad start etc.
It's like the suspension,you can have a better suspension but you can customize it
Also,i read somewhere here that some will find the ultimate setup and then all the time devs spent on that is gone.As said,the customizing will have different effects on your car and not only that.Different tracks require different setup just like suspension.In a dragstrip you want all the power you can have just because its a straight,but on a 30 lap you will need efficiency,because if your car overheats,your out of the race.
But first,other things should be done in LFS like realistic damage.
I'm not saying this should be done now,all things will be made sooner or later!!