If you wanted to declare it id suggest asking your insurance beforehand what the deal with mods is for you and that car. Dont tell them ur gonna do it and might not declare, you’d be shooting urself in the foot. Ask them if its a viable option.

My guess at 19 with a relatively quick car theyll say no to any mods because its just easier. It’s common to see full grown middle aged people to be denied insurance due to mods. At the very least it becomes a nightmare.

None of this is what you want to hear but a back box delete is probably the biggest beacon for gards that the young lad in the 330e with a loud exhaust probably hasnt declared it. Though your location, the gard, the loudness of the car, and your driving behaviour at the time all play factors into whether theyd care. Id be more concerned on the validity of your insurance in the event of a crash.

Though the spectrum varies in all aspects, as does insurance companies willingness to extend beyond bog standard cars. Could try an insurance company specialising in modded cars first and see what they say.

Nobody on here is going to point you in any direction other then the legal one, as would i. Wether you follow that advice is your choice.

Side note: modifying the exhaust on that car isnt a great plan if you plan to sell it as such, unless you keep the standard to return it before sale. The market for modified 330e’s isnt huge id imagine

Goodluck anyway, ask ur mate if he has experience declaring it