See your problem and say you just ignore the warning message it is just for Career and some MP lobbies upgrade it to your hearts content will not be a problem at all the Class Restriction is just for the homongiesed part of the game.
Welcome to the worlkd of upgrades and tuning for tuning tips have a look in the tuning section:+1:
You can upgrade your S750 car up to S800 or beyond if you like - it’s just warning you that you are not going to be able to use it in races where you must stick to the homologated limits set by the game.
So in single player that Ferrari in whichever group it fits in to - all of the other cars must be ‘homologated’ to the S750 limit, with the set tyres, tyre width, HP limits etc that make up that little grouping. It’s a way for the game to balance the field and try to make more of the cars competitive with each other. So naturally if you say fit race tyres and everyone else is running street tyres, you would be way more competitive - so it won’t let you race in that group any more.
The S Class hopper online though, will allow you to run any car so long as it’s in the S range - you could take a D class car and build it up to S class and run that against whatever everyone else is running.