That is not entirely true, as long as you don't disclose it to them they have a legal responcibility to cover you. If you get in an accident they will cry the blues about how you didn't disclose it and so on, but you can just respond with "well I didn't know I had to disclose it". There is lots of precidence cases for this type of issue, however if they even get a whiff that you have a Turbo they will cancel your policy immediatly.
As an example, if you accidentally even mention a room-mate they seriously freak the f#ck out if they find out and want all of the information about his/her driving history because they become an additional liability.... however if you don't tell them your car and that person should they ever drive it are still covered.
Insurance comapanies are whiney little bitches when it comes to coverage, but if they try to wipe thier hands clean even though you had any kind of insurance through them over a small disclosure thing the law will step in and put them back in line.
If you have any concerns at all I would contact a Lawyer and just verify that the presidence is there should you ever need it.
Be aware that if something ever happens you will go right onto a blacklist for most places though
