Car stolen, compensation for upgrades?

I wouldn't tell them about performance upgrades if you have not already reported them. Most insurance companies have a clause in their policy (which you should have read and signed) which exempts them from paying for an accident or vehicle that has been fraudulently misrepresented to them. They wont pay for the upgrade, could fine or charge you with fraud, and could even refuse to pay for the car, all depending on your specific policy and company.