The Nitrous physics in this game are so far from realistic it makes me sick. For example, if you have a car with say 500hp and it receives a 100% shot of nitrous that doubles the HP on the car thus making it 1,000hp and you don't just get this for a short amount of time you get this all the way from 0rpm to redline and that isn't realistic. In real life you'd only get an acceleration boost for a short amount of time, if people tried to do what the nitrous physics in this game do then you'd most likely blow your motor especially if you miss a shift.
This isn't against you Shells, as I agree with most of what you said, but I'm highlighting this to explain the mistaken reality of nitrous use in the game.
The nitrous physics indeed give a boost of horsepower throughout the entire powerband, which physically is accurate to real life, the difference being the amount of horsepower. Normal nitrous, used in shot increments, vary in horsepower ranges from 25 all the way to 200, 250, 300, and higher. The problem is that nitrous shots exceeding 100 horsepower cannot be used constantly like this game would lead you to believe, but only under heavy throttle so fuel and air consumption can match the extra nitrous injected. If nitrous is used during a shift when RPMs change, and there's excess nitrous trapped in the combustion chamber without proper fuel/air mixture, it will could cause pre-detonation that would destroy your piston or valvetrain, or both.
If you have a fuel injection setup with a wet injection or direct injection system, the spray can be variable with computer control according to throttle input as well as a purge system to get rid of the excess nitrous trapped in the cylinder. That system, however, doesn't allow you to simply "spray & pray" like is allowed in this game, but instead injects lower amounts during lesser throttle input and increases with throttle.
PD has decided to go with a dry, push-button setup whether for ignorance of how the gas works, or because they don't care, and allows you spray any strength jet at any time or RPM you want without any change in rate of horsepower. This is not only completely unrealistic in every way, but everyone using nitrous in this game would have to rebuild their motor after each race. The argument of using nitrous in lobbies because "people use nitrous all the time in real drag racing" doesn't cover the fact that nitrous is infact overpowered and not regulated for reality in any way.
All of this information doesn't even include the fact that a block needs to be stout enough to handle that jet of horsepower, and I doubt any block in this game, especially the JDM cars in this game that start with under 300 horsepower stock, would be able to handle a 400 or 500 shot of nitrous without throwing a rod through the crankcase. Just to handle a jet of 200 you would need fully forged rotation assembly and still would recommend titanium valvetrain.
TL;DR: Nitrous in this game isn't realistic.