Getting an appraisal is a worthwhile thing to do. The appraisal value does not specifically note performance modifications even though it accounts for them in the final value. Insurance companies have a problem with performance altered machines. They don't want them. hahahaa Can we say higher risk? Now, if you get the blanket appraisal amount and show your agent the certificate, they just increase the insurable amount to match the appraisal. Yup, your premium does go up. Why wouldn't it? More insurance, more premium. The trick is to keep the company from knowing you just took your 450 horsepower car and made it a 700 horse car.
Cosmetic changes are never of big concern to insurance companies, only performance modifications.