mainly depends on the insurance company, adjuster, and the state you live in. some are very friendly with mods, others are not. I have a rider on my policy that covers ANY part that's bolted into the car, from stereo to brakes to engine mods...
Other companies like Farmers or state farm will void your policy if they see anything that's not 100% street legal on it... that actually goes for any company though- legally they are allowed to deny you coverage if you bolt something onto the car that no longer makes it street legal. If you can pass a state inspection with it, they will usually cover you, but if you can't pass inspection, then the insurance is allowed to deny you coverage because you are willfully driving a "faulty" car that is illegal to be on the street. Thus, they consider you a perpetrator at that point and the burden of proof is on you to proove that those parts did not cause the accident or ticket or claim or whatever. (and good luck getting past that when the cop and insurance company are both on the same side!)