That's not really how it works
Insurance is based on statistical models. Their data will probably show that wrapped cars cost them more in clams. Possibly because of a particular demographic that typically wraps cars etc. The cost of replacing the wrap after a repair is unlikely to factor into it very much. If you don't tell them (and if the assessor notices when he examines the wreckage) then your insurance is void. End of story.
Also different insurance brands put different weightings on various factors in order for the underwriters to balance out the risks they are taking. It could be that they don't want wrapped cars bundled in with the mid-40s careful parents group of policies so they price them out. You may need to find the insurers who take on the rich boy racers group. They might charge much less extra for it being wrapped, but may charge more for other reasons.
Or maybe people who wrap cars are a small group who are typically willing to pay more so there's no competition.
Also there is no such thing as under-insured. Something is either insured or it's not. The idea that if you insure something for half it's true value then you can get half the payout is completely false. If you insure something for the wrong value or the wrong description then it's not just the replacement payout that's affected - it also affects the assessment of how likely it is to be stolen, how well people look after it and all sorts of other factors.