If you don't, you run the risk of the insurance company only covering the NADA value of the car w/o enhancements. Some people have a lawyer on speed-dial and would have the desire to fight the company, but it's far easier to simply disclose additional mods on the policy. All insurance companies act differently, but with mine, I simply declare I want to insure the vehicle plus $x,xxx in mods and they factor that into the premium. You will pay more, but they will cover your mods in the unfortunate event of a collision. I would avoid specifics on the extra $4,000 coverage for a turbo, as they associate turbo with speed and speed = danger... and danger is baaaad, mmmmkay?