The brakes issue is that the car can then stop much quicker than normal, which then could catch out people behind you and they then drive into the back of you. Okay, so that's their fault not yours, but if they're insured by the same insurer as you then that insurer has to pay out twice still
Tbh, I think that's a load of old cobblers and it's more the fact that the car is modified and therefore more likely to be stolen, as opposed to anything else.
They should ask the company they're purchasing from. If they've followed due diligence, then they cannot be to blame.
And Europe, which is why there's a huge problem with Johnny Foreigner being done for not having insurance in this country. They don't realise that it's different over here.