As PP said, lying to insurance is like giving money away. You still continue paying the monthly premium and when they find out you lied about an aftermarket exhaust, they just walk away.
I have always been straight up to my insurance providers about add ons like aftermarket stereo in cars, the exhaust on my wife's bike, etc. In all cases, all they wanted was proof/receipts showing the cost of the aftermarket items, and that's it. They added lines to my existing agreement such as x amount in stereo, y amount in exhaust, etc.