Think about sounds you hear when you travel on a car, then you notice that tire and wind noises are quite loud compared to engine noise, imo, default settings for road cars do have this level wrong as if you cruise 2000rpm and 100kph there is more engine noise than any other.
I think that many that have tuned engine sounds have gone to same error and that is why they get clipping, reducing exhaust volume should help, just need to adjust other levels accordingly.
Full throttle sound level of course is much louder and at 100kph on some cars it easily overcomes wind and road noise, but when doing 180kph there is not many road cars that have engine noise so loud that it would be more dominant than wind and road noise.
Here comes also limitations of current sound engine setup, wind noise does not increase enough with speed, you can adjust volume levels from this menu, from other in game settings and from speakers, so that it is fine at higher speeds, but at low speed there is then too much, or then you don't get enough that noise on high speeds.
Race cars, I really don't even care how it is, there is so much more things like health issues coming into play that it does not matter if those are rather unrealistic in this matter.
Good answer from GP4Flo about clipping.