The nitrous upgrades have become more grounded in NFS Heat with a fairly simple upgrade path.
The higher the level of nitrous, the bigger the overall capacity of the system will be, as well as the boost increase from the engine.
The strategy when upgrading nitrous comes down to choosing between either installing an additional small bottle or replacing it with a bigger single bottle. The choice between multiple small bottles or one larger bottle is entirely yours and will be dependant entirely on your own playstyle.
Having multiple small bottles gives you better flexibility on when to activate each one, but it does have a lower power output compared to the bigger single tank option. The downside to using the bigger tank comes down to the fact that, when used, it will provide an increase to power until it’s empty. If you choose this option, then you’ll want to be sure that you’re hitting the fun button at the right time. You don’t want to be receiving your increased power output just before turning a corner, for example.