Craig wind yer neck in, next thing you want to tell them you putting nitro+ in yer tank, that ups the performance doest it, dont sit there and dare say people like me shouldn't be on the roads on this forum cos I know you wouldn't dare if I was in front of you. how many cars do you think get remapped then Px in for a newer model, the dealer doesn't know, you dont know what do you do then, I know lets all take our cars to our local remap place and tell them our car feels a bit frisky, can you please plug it in and see if our mapped have being played with cos I need to tell my insurance company.
Final sentence removed due to language - b1g_dav3
No need for language like that on the Forum and it won't be tolerated, please keep this in mind for future posts.
Nitro just has additives to keep the motor clean, and adds a tiny, tiny bit of performance, a lot less than 5% I'd imagine.
Generally speaking you should declare any modifications, although as you said most don't. The trouble is when the insurance company see's who you are (i.e. if you're considered a high risk group) then the insurance company are likely to look very carefully at the vehicle, especially if bad driving was a factor. They'll look at air filters (replacement panels can often give the game away), and companies can sometimes tell if you have played with the engine.
For example, there are often tamper devices on the ECU itself, so things like stickers for example that may be broken and not replaced. There's also sometimes codes on the ECU that are saved to indicate these things. Good companies usually edit the stock map rather than flashing a custom one on top so that all features are saved and it's harder to tell, but there is usually a digital finger print.
That said, the insurance company might take one look at the car and pay out. Just depends on how vigilant the company is really.